Battle: Los Angeles/Headscratchers

They also appear to want fairly intact infrastructure, and don't want to risk damaging the environment further than it already has been. Dropping rocks from orbit would royally fuck up the landscape, the infrastructure, the environment, and the water they're after. That alone would be good reason for them to not want to drop rocks on us from orbit. If they wanted an intact infrastructure, then that means the scientists in the movie guessed completely wrong about their motivations. The movie scientists speculated that because they were wantonly wiping out the population and the buildings, that meant they were only interested in our natural resources (i.e. our water). Another issue is that, if they're as desperate for water as they are indicated to be, it might be a massive logistical problem just keeping ships in orbit. They'd have to constantly be sending refueling vessels to carry either water or power capacitors up to any ships orbiting the planet, for questionable gain considering their demonstrated weapons capability. It would also explain why they're not using nuclear weaponry. They don't want to risk irradiating a planet that they want to take and hold and colonize, especially if they're in such dire straits - at least not right away. If I were in the aliens' shoes, I would hold off on WMD use unless absolutely necessary, because I would want the real estate as intact as possible so I could use as much of it as possible. In fact, I believe that the most viable human military response to this scenario you're presenting, particularly among the US military, is that they'd start spamming thousands of drones, given enough time to prep and establish factories. No need to waste human lives if they can just crank out thousands of drones to counter enemy drones and launch ground-strikes. This may also be further evidence for the theory that the aliens are either desperate or are some sort of equivalent to a Third World militia. They may very well be their equivalent of a gaggle of militia tooling around in technicals with .50 cals and recoilless rifles instead of a fully-kitted military. Unless you've got some sort of non-vibranium element stored somewhere, it's probably better NOT to wear anything for unrestricted movement so you can avoid the shots instead of futilely deflecting/blocking them with armor.
 * How did the alien command center get so far from the ocean and still be covered with rubble/underground?
 * The aliens were moving it during the battle. Fog of war will royally fuck over any attempts to gain meaningful information, especially in an urban battlefield where the enemy has aerial superiority.
 * I was under the assumption that they built it there, or just a big chunk of it. If it was a colonization effort they might have needed to make some of their stuff on-site.
 * I thought it was obvious: It came down with the landers, and then lifted itself out of the sea and buried itself into the ground. It was obviously flight-capable, and we're told at the mission briefing that there's something big at the center of each lander.
 * Having the alien officers be well over a head taller than their grunts is pretty stupid. May as well have them hang a sign on themselves that says "shoot at me".
 * Having the officers blend in with the grunts is a fairly new development for (Earth's) military. Officers in bright coats and/or on horses isn't unheard-of.
 * There are definite benefits to having officers stand out like that: 1. They can get a better view of what's going on on the battle field. 2. The regular soldiers can see them more easily to get orders.
 * Alien officers show themselves twice in the entire operation (once on the rooftop, and once during the engagement around the command center). For the most part they stay out of sight, and only appear once an area appears to be secured. The "officers" themselves are probably upper-echelon command troops; the one on the rooftop looks like he's scoping out the terrain of the city like any good major/colonel/general would. The combat squads that the Marines encounter in the movie do not have differentiated officers, so likely their equivalent of NCOs or Lieutenants are indistinguishable from other infantry.
 * During the Final Battle, there actually is a commander alien firing his weapon at the marines. Maybe he wasn't even supposed to be on the front lines, but hey, there were humans to kill.
 * They might not have any choice in the matter of size. Maybe they grow until they die, so the older, experienced guys tend to be big. Maybe the smarter race/gender/nationality is also the bigger one. Maybe the royal bloodline gets all the best jobs and grows bigger on higher-grade food.
 * At the end, when the surviving squad members reequip and head out, saying they "had already eaten breakfast", and the commander lets them go. First off, we have no proof (other than The Law of Conservation of Detail), this squad has eaten ANYTHING substantial in almost 48 hours. I'd was surprised the commander didn't order them to clean up and eat. They had been the closest to the enemy and the enemy's command center and thus had valuable information. These guys needed to be debriefed, so their information could be analyzed - heck, Elena got alien guts in her mouth - she really should get a medical check.
 * Marines carry at least some MREs on them when operating in hostile environments. They probably ate while at the destroyed FOB, and there was food inside the store where they were hunkering down in preparation for the bombing. They could have eaten at any point where they weren't moving (yes, The Law of Conservation of Detail is in effect here. We can probably assume they were dumping, pissing, and napping when we weren't looking either) Also, from what I gathered, there had been some passage of time after they returned, probably enough to give a report on what they saw and get a medical check, but not enough time to bother cleaning up. Besides, cleaning up and eating hot food is a low priority when combat is underway. For example, after the Second Battle of Fallujah, a lot of the Army and Marine infantry spent quite some time without hot food or cleaning up. I've read accounts straight from the soldiers and Marines serving there that they were ludicrously filthy and hungry, and this is while sitting outside the city, in a FOB, after a week of off-and-on combat. Tech Sergeant Santos probably wouldn't have time for a serious medical check - just a medic giving her a once-over, certifying her as able-bodied, and moving on to people who are more seriously injured, of which there will be no shortage.
 * In fact, a Marine at the beginning of movie is more concerned that someone took his Cheese Tortellini MRE than anything else.
 * Marines and Soldiers "strip" MR Es, breaking open the package and taking out the main course (and a few other bits) leaving behind things like the gum or parts they don't want to eat. Each one is 2,000+ calories so if you expect to be only out fighting for a day or two you want to travel light.
 * Or it could be seen as a figure of speech. A way of saying that the job isn't finished and that they didn't want to waste time on something they could easily due once Los Angeles was secure.
 * A medical check wouldn't do any good here if some of the diseases are alien because we wouldn't be able to recognize it, thus it would pass through all screens and tests without anyone noticing.
 * And they must sleep as well, human body can't work properly longer than 24 hours without sleeping...
 * Lots of sleep is not necessary to remain in fighting condition. Most soldiers can easily get by on a few minutes here, an hour there, and so on. This is pretty much a requirement for anyone going Infantry, and most infantrymen are adept at sleeping quick and waking quick.
 * During the beginning, one of the soldiers drops in on doc's webcam chat. He see's the lady he's talking to and says "Man, is that your girlfriend?" and remarks that she is hot. Doc explains it is his sister. The soldier becomes apologetic. I know this is for the Rule of Funny, but why is thinking a guy's sister is hot worse than thinking his girlfriend is hot?
 * He's apologising because he mistook the guy's sister for his girlfriend, not because he thinks she's hot.
 * Being told your girlfriend is hot is a compliment. Being told your sister is hot is a threat to your family.
 * Especially if the potential suitor is a low-status grunt. Men tend to want their daughters and sisters to marry well -- doctors, officers, successful businessmen, etc.
 * Pretty much. Several of my coworkers have been told that their siblings are attractive, and immediately went into Dude Not Funny territory.
 * It varies according to culture, but the suggestion of incest, even an unintentional one, could also be taken this way.
 * The scientists speculate that the aliens are here to steal our water, because liquid water is found nowhere else in the solar system. Couldn't the aliens just steal frozen water from comets, or from Jupiter's moon Europa, and then melt it?
 * Its not just liquid water, its also speculated that they're after the chemical composition of the water (likely salt water is of particular value). Also, it would be easier to acquire water from a planet with temperate conditions than a frozen one. There wouldn't be any resistance, but the environment on Europa is not terribly conducive to supporting life, and comets don't have anywhere near as much water as Earth's oceans.
 * While a single comet has less water than Earth, there are a lot of comets. The total amout of frozen water estimated to exist in the Kuiper Belt is staggering, far far more than the meagre amount of water covering Earth. Pluto alone has hundreds, if not thousands of times as much H2O as Earth's oceans do. And while it's true that conditions on these icy worlds aren't conducive to supporting life, these aliens can survive interstellar travel. There's no reason they couldn't build more living quarters for themselves in space while they sucked the Kuiper Belt dry.
 * I don't think "Aliens can find water elsewhere" (which is true of several other scifi films) fits here. These aliens seem intent on not only using the water but setting up a home on earth.
 * In fact, they probably did drain the outer planets, moons, and comets dry, but our sensors are so unsophisticated we wouldn't have known about it.
 * I suspect that the aliens' strategy was probably to hit Earth first because it is a habitable world they can operate from, and once they'd established themselves there, they could move out into the rest of the system and harvest the water supplies there. It also fits with the "desperation" angle that is being suggested and strongly hinted at in the movie. They're not going after Earth because its the only resource available. They're going after Earth because they need a staging area where they can base their population on, establish their civilization, and then move out from.
 * The key word is "speculate". The aliens' primary goal could be anything -- religious conversion of the heathen Earthlings... saber-rattling to impress the Klingon Empire... harvesting of highly-addictive human spinal fluid to sell on the Hutt black market... instinctive "survival of the fittest" winnowing of young males before breeding season... lucrative pay-per-view contracts... anything. The water could just be a means in service of that unknown goal.
 * Depending of if you buy into the background stuff they're using it as fuel for vehicles at least (either for hydrogen engines or in reactors)and few things in existence are as fuel hungry as a modern mechanized army. So room certainly exists to suggest the entire "stealing water" thing is just a byproduct of them needing to fuel their huge invasion force, not the end in and of itself.
 * If the aliens are out to take our resources, and don't give a damn about leaving any of the native population alive, why are they even bothering with an invasion? Why not just nuke us from orbit? These are creatures with a technology that allows them to cross interstellar distances, they could certainly do much more damage than they are shown doing.
 * There's a lot of assumptions in this question. You're assuming that just because they managed to travel interstellar distances, that they're capable of putting ships into orbit, which goes against the tech base and behavior shown. Their mode of transport indicates that they were specifically using one-way transports. All we can really conclude is that they can't (or won't) put ships in orbit for one reason or another; maybe they are dealing with technical issues, or the invasion is underfunded, or the forces they're deploying are the remnants of a massive colony ship that ejected after a catastrophic failure and set out on a one-way desperation trip to the nearest habitable planet (see the "The aliens are desperate" entry on the WMG page).
 * Unless we assume that their spacecraft were launched from another star system with such precise aim that they were able to hit their landing sites on Earth completely unguided, their spacecraft had to have course-correction capability. If they can course-correct, they can orbit, period. The delta-V necessary to brake from a modest interplanetary speed (which those "meteors" couldn't have been going much faster than, if they were to survive the trip through the atmosphere), to a low Earth orbit speed, is trivial when compared with the delta-V for their entire mission -- even assuming they didn't have some FTL means of crossing the interstellar gap entirely.
 * Again, see the "Aliens are desperate" line of thinking here. The aliens' behavior is much more consistent with a force that is strapped for tech and weaponry and is in desperate straits than a typical invasion fleet. It would explain why they're not putting ships in orbit. In fact, going by the alien's comparatively low-tech weaponry, it is quite probable that the aliens don't have any weapons that could be effectively used from orbit; their heaviest weaponry are cluster-rockets, anti-tank missiles, and drone fighters. They may have judged it pointless to keep craft in orbit if those craft can't do anything while they're up there. If that is the case, then we're probably dealing with the aliens' equivalent of a military made up of guys driving twelve-ton trucks, technicals, and motorcycles: a ramshackle, slapdash force of poorly-outfitted militia.
 * You're assuming the flight was made entirely on pure thrust, nothing supports in anyway. If they used some sort of FLT drive the landers could be less ships and more ballistic missiles with almost no real maneuvering to speak off. In fact details support this as the landers do not appear to be the sort of enormous craft you'd need for interstellar flight and they're only spotted at best weeks if not days before impact. The drive plume of an interstellar drive would be enormous and would probably be spotted approaching the solar system with likely years to spare, since during deceleration ti would be pointed RIGHT AT US. My personal feeling is that the landers were probably deployed via some sort of fixed FTL drive and were effectively ballistic. Likely built as cheaply as possible in order to allow a massive swarm that could cover numerous landing sites.
 * The second paragraph above is directly supported by the movie; it's noted that the aliens aren't just draining the oceans, but using local sewers and canals to move local water back behind their lines as they advance. It may be overkill to call their vehicles gas-guzzlers, but their technology clearly isn't magically efficient just because it runs on water instead of gasoline; they do not have Easy Logistics by virtue of having interstellar travel.
 * The first lesson of any Theoretical Sociology course is that no civilization is going to act exactly how you guess they will. Speaking of civilization, has anybody here ever played the game, Civilization? Technology is not a steady progression down a set path. The aliens may not have ever encountered fossil fuels, or, going by the theory of desperation, may have exhausted their fossil fuels so long ago that they no longer know how to work combustion engines or abandoned it as a (not necessarily obsolete, but with none o the required resources, it becomes a) pointless technology. Also, as advanced as they may be, keep in mind that some advancements may have just never occurred to them, or were wrapped up in Development Hell caused by Executive Meddling back on their homeworld. Think of all the technologies we might be able to have right now but don't because of ethics laws interfering with the advancement process. Now, those surgically grafted weapons might put up an argument for their ethics even existing, but the first lesson of any Theoretical Xenopsychology course is that most likely, an alien's idea of logic or common sense has little to no correlation to what we call logic and common sense. This is part of what makes them Starfish Aliens. Keep reading the "water fuel" question below for more on this.
 * The aliens supposedly use water as fuel. In the 24 hours since the alien invasion started, the sea level dropped noticeably all across the Earth. Even if we assume that this noticeable drop was only a centimeter, at that rate Earth's oceans would be drained completely dry in a couple hundred years. How has this alien civilization not collapsed under its voracious water appetite already?
 * Good point. There's a WMG guessing these guys were pretty desperate when they arrived. Maybe use will slow down once the invasion is over, plus "a couple hundred years" should be plenty of time to refuel their spacecraft and head back out into space.
 * Yeah, the desperation angle is the most logical, and it most strongly fits with what we've seen (relatively limited tech, lack of orbital assets, soldiers with weapons grafted onto their bodies, etc). They want a habitable planet; they don't care if they're going to consume all of said planet's resources in a century when they're concerned with survival for the next decade.
 * There's a wiki about alien species out there on the net, and it has a decent-sized page on these "Blaa" (get the name: battle los angeles aliens). It also had some references to interviews with the producers that inferred that the aliens' planet is actually a war-ravaged shithole, so, to sum up: their civilization likely did collapse under their voracious appetite for water.
 * Also consider water as a fuel in the vacuum of space. It has it's own oxygen mixture and it likely fuels their life support systems. Hell, they were breathing our air. This troper is impressed by the writers' ability to make aliens that are so much Not So Different that they approach the Uncanny Valley on a psychological level.
 * It's possible that their non-military use of water is much more efficient. Once they wiped out humanity and set up civilization, the remaining water might last a lot longer.
 * It's also fully possible that they were stockpiling it somewhere and somehow. That way, they have a reservoir of sorts if their method for harvesting the water is destroyed, and they have at least some water captured if the whole invasion goes south and they have to escape.
 * Likewise we don't know how much of the water they consume as fuel can be reclaimed and used again when it turns to vapor and rains down. Also, a couple hundred years is probably more or less the time frame that they want to stay anyway before moving on to the next conquest.
 * Why is the alien command and control centre on the ground at all when it could be orbiting the planet like a communication satellite in geosynchronous orbit, giving much wider coverage as well as relative invulnerability? OK, we know it has to be on the ground for plot reasons so the marines can blow it up, but I didn't have to wait till I went to the fridge after the movie before I thought of this one.
 * See above discussion on keeping their spacecraft in orbit. The biggest problem is likely keeping the thing fueled.
 * Also, if it's in orbit, then it has no cover and can be seen very easily. That makes it easy to launch ballistic missiles at. Human militaries might not know what it is, but they can guess its important and stsrt chucking nukes.
 * Water being used as a fuel period is a bit unusual chemically. It's a very stable substance and isn't prone to combustion or most chemical reactions. While it's possible that the use of "fuel" could mean that it's being used to absorb ions, there is very little that it can physically run.
 * Water itself is not a suitable fuel. But, with the magic of electrolysis, the chemical bonds can be broken a yield hydrogen and oxygen. Hydrogen is the fuel, and oxygen is needed for combustion so that you can get energy from it. They probably have have power generators and such, but from a logistical standpoint it's easier to make fuel to distribute to all of your ground troops then to try and spend the resources and time to build an energy grid. It also probably for the same reason why we use gas in our cars instead of miniature nuclear reactors, safety with something that's not easily replaced.
 * They would still need to power the electrolysis somehow though. As is stated below, ruling out fusion given the inefficiency, there doesn't seem to be a source to run the conversion process.
 * Water is also a very convenient way to carry hydrogen atoms around at room temperature. If the aliens have nuclear fusion technology that works off of ordinary (non-heavy) hydrogen, it might make practical sense for them to carry their fusion fuel supply around with some extra oxygen atoms attached to them. (Of course, given the rate at which Earth's sea levels dropped since the aliens arrived, that represents far far more fusion-fuel than would be needed to power every city on Earth several times over. If they are using water for fusion power, their fusion reactors must be terribly inefficient, either failing to fuse over 99% of the hydrogen fed into them or wasting enough heat to boil the Earth's atmosphere.)
 * Well, its possible they have a different method of nuclear fusion than we think of on earth, and the process they use requires quite a bit of water to use
 * On Earth, the biggest problem with nuclear fusion is achieving a high enough temperature and pressure. The aliens' method might require some nuclear catalysts, one of which is water.
 * Attacking America as a whole. The aliens are smart enough to know where the major cities in the world are (and are capable of dropping within miles of the coastline). It's tough to imagine a reason why they didn't pick smaller cities, set up a base on the surface, extract water, then fend off earth attackers.
 * OTOH our military is spread thin between Iraq and Afghanistan, and our military tired out.
 * They want to wipe out humanity. It makes more sense that they'd attack population centers first to kill as many humans as possible and do as much damage to human infrastructure as possible before a counterattack can be initiated. Also, inflicting damage on the most powerful political, economic, and military entity on the planet can only benefit them. The more damage they inflict to the superpower most capable of projecting military force across the globe now, the less they have to deal with later. The question isn't why they attacked the US - the question is why they wouldn't.
 * Because it's heavily inefficient troop wise to attack heavily defended areas. Even IF the US was a desired target, it'd be easier to work around taking fewer troops on the initial attack and forcing the US to make a move. A common military rule of thumb is 2-1 casualties for the aggressors. By forcing the US military to attack, more casualties can be forced on the US per casualty of alien. Soon, while it is complete unseen how many more forces the aliens have aside from the initial wave, the aliens can complete a slower, but steadier, wipe of the world (as, having some survivors and some land is better than having few survivors and lots of land) with much better technology (fighters proved completely ineffective). And if the aliens really don't have any more forces, owning the planet for themselves to start seems like a lofty goal (granted, there are bigger problems with the alien intelligence than overachieving, so it may not be out of question).
 * This strategic approach is pretty much doomed to failure. The aliens have thirty million troops total. The humans have seven billion and growing. The aliens can't win a war of attrition, especially if they follow your suggestion of forting up and consolidating their position. They could trade a hundred human lives for one of theirs and still lose in the exchange. Doing so gives the humans time to recover, arm up, mobilize, and gear up for war. World War Two should have taught us everything we need to know about what happens if you leave one of the most powerful industrial centers on Earth enough time to arm up. The only reason the US is in any state to be invaded is because of overseas deployments coupled with the fact that the country currently isn't on war footing. Give the United States six months, and the amount of war materiel they'd be able to produce, coupled with the industrial capabilities of other countries like Russia and China and deeper, landlocked countries far from water, and humanity will be able to re-enact the Eastern Front all over the invaders with sheer. The only hope the aliens have is to hit first, hit hard, and knock out the strongest military-industrial superpower before they can catch their breath and turn that capacity on the aggressors.
 * The thing is, the aliens didn't just attack America. We get specific mentions of China, England, Germany, Brazil, and probably others I missed, being attacked. With such a widespread attack, humanity loses all chance of organization (see something those locations have in common, besides their coastlines? They are major world powers. Brazil is the most powerful country in Southamerica together with Chile -which, as nothing but coastline, probably got attacked as well-, Germany is one of the biggest economical powers in the world, and right in the middle of Europe, Britain has nukes, China is the other superpower with the US, etc.) During the initial invation there's a mention of the UN calling an emergency meeting to discuss how to share resources to fight the aliens. Wouldn't this be much easier if the big players, specifically the house of the UN, were left in peace? Food for thought, there's a short story online about some desperate aliens invading the Earth during the Cold War. They attack the US and manage to "shock and awe" the country into submission very quickly, but since they don't touch Russia (hoping the political differences mean they would let the US be destroyed) they see their invasion thwarted under an unending barrage of intercontinental missiles and the defensive effort of a worldwide alliance. This would be much harder to organize if every powerful force was too busy fighting for survival to rally the rest of the planet.
 * It is worth noting that there is at least some attempt to rally a worldwide counterstrike; a reporter early on mentions that the UN is calling an emergency meeting in an attempt to share resources to counter the invasion. In light of this, eliminating as much of the US' military and infrastructure infrastructure and capacity as possible might make sense, as they'd need to knock out the industrial capability before humanity can get back on-balance and counterattack.
 * Without reinforcements, holing up and waiting for the enemy to come to you is less of a way to win a battle, and more of a way to delay losing one (think of most sieges in history). Besides, the element of a surprise attack would be wasted if they used it to steamroll some defenseless country they could take at their leisure any other time. If you're going to start a fight with a free punch, swing it where you'll do the most damage (think of Pearl Harbor).
 * On the strategic scale, if you're engaging in an offensive war, and instead you're holing up and letting the enemy attack you, this pretty much synonymous with "losing." You're surrendering momentum and initiative to your enemy and letting them make the choice as to when and how to attack. If you don't attack, you give the enemy time to fort up as well, and to mobilize more resources against you. The main reason the aliens were able to achieve as much as they did was because they hit with surprise against unfortified cities, against a military that was not in a defensive posture and ready to engage in war on their home front. If they went up against a US military that was dug in, with mobilized reserves, and a mobilized economy directed toward a war for survival, the battle would be a hell of a lot harder.
 * It's obviously possible that this troper missed some mention during the explosions, but why did the military assume that the invaders didn't have an aerial force? If these creatures are advanced enough to cross space and invade another planet isn't it safe to assume they'll have a pretty substantial air force as well?
 * Because there were no aircraft in the air. The military didn't seem to be assuming that they had no air force at all, they seemed more to be assuming they could hit the enemy before they could get their aircraft in the air.
 * "They are a ground force enemy only. They have no aircraft, so the Air Force is going to tear them a new asshole." This indicates that the military suspected the possibility of air forces but that the aliens had no air force at that point in time.
 * Nantz has been in the Corps for twenty years but is only a Staff Sergeant.
 * ....and? The Marine Corps doesn't promote non-commissioned officers aggressively like the other branches, and he may have been passed over for promotion often, or refused promotion for various reasons. I personally knew a Staff Sergeant who had been in the Corps for twenty-five years.
 * And the primary "B-Plot" of the movie and the biggest nugget of backstory for Staff Sergeant Nantz is he was in charge of an operation were several men were killed. That might have killed his chances for promotion.
 * Not really, because we see that the only people who looked down at Nantz were people who either had a personal grudge (ex. Corporal Lockett, because of his slain brother) or Lt. Martinez, who was eager to show that he could lead in combat. They all learn that Nantz didn't receive that Silver Star for nothing. You get the feeling (especially in the scene where  that he wanted to get away from the horrors of war, especially since he was so good at it.
 * We don;t really get to see much of nantz's treatment outside of a very small circle. But note how the Marines reacted to Martinez's death. The first thing they thought of wasn't "Man, the Lieutenant died like a hero." or "Man, it sucks to be Nantz now." Their first response was "Did Nantz just leave the Lieutenant to die?" That kind of immediate reaction is not the one you get when everyone thinks you're a reliable hero. Its the kind of rep you get when everyone thinks you're a coward or a heartless commander who callously sacrifices his men.
 * What were those things in the water in the sewers at the end?
 * Alientech. No explanation is given.
 * What was the deal with the alien "assault rifles"? They did not seem to do any puncture damage, and only minor incendiary damage. They were inferior to our weapons, yet the aliens have FTL? It doesn't seem to make any sense to say "Oh, they're not warlike, so they don't put time into making lethal weapons." either because a non warlike race does not do what these aliens are doing and invade.
 * Incorrect. We see those guns go straight through Marine body armor like it was paper (freeway battle, the shots that killed the Marine radioman and the Army riflemen) A glancing blow from one shot set a Marine's helmet on fire and burned him through the helmet so badly he was blinded; during the initial ambush Guererro takes a glancing shot across the top of his helmet, and his face is seriously burned. A single glancing shot gouged about a three inch hole in a concrete pillar during the final battle. Every time someone's been hit by a shot they've either been thrown off their feet or spun around with a significant amount of force; for example, during the initial ambush, you can see one Marine get hit by a glancing shot that just barely grazes his vest, and he is literally spun around and sent sprawling by the force behind the round. Later on, when Lenihan is alone and shoots it out with the alien in the pool, it manages a grazing shot on his backpack which throws him onto his back. No modern assault rifle hits with that much kinetic force. The round that killed Mr. Rincon left a fairly large entry wound and burned flesh around the impact site, and direct torso hits by the aliens' weapons consistently kill anyone who catches a shot by them. Most of the aliens seem to also have some kind of rocket-grenade launcher that can destroy an M1 Abrams in a single hit on the frontal armor (multiple direct hits from Javelins, Hellfire missiles, and 120mm APFSDS rounds have proven ineffective against even damaged Abrams tanks, by comparison). The aliens' guns are quite effective at their job, and are at least comparable to modern small arms.
 * Not 100% true with the Abrams we see in the film. While the turret was facing the Aliens, the front of the tank was not. They were also firing on the tank from an elevated position, which exposes the tanks weakest armor to incoming fire. We also never really see what strikes the tank, but a safe bet can be made that it might have been that walker you see only a minute or two later. In all likely hood, that mech was the equivalent to the TOW system, or a recoiless rifle. IE, it's job was to destroy armor, so I think it's a safe bet to say that these aliens may have encountered something like tanks in the past. Not only that, they do deploy a type of fast-attack vehicle in the final battle. So even if they them selves never fought a tank, they were prepared to fight them incase they did. You always assume your enemy has the same thing you do, and adapt from there.
 * It definitely was not the walker. The weapon used against the tank was a single projectile almost certainly the same as the "grenades" they're shown periodically firing. The walker fires multiple missiles every time it shoots.
 * Unless they use multi-missiles for cases where it's dealing with infantry, and a single large missile for cases when dealing with tanks or bunkers. We do the same thing, hell, the Abrams rolls out with 42 rounds, 20 Sabot, 4 Canister, 10 HEAT, and 8 HEF. Don't quote me on those numbers though, I just know that MBT don't roll out with just one ammo type. And for all we know, the shot could have acted like our Javelin, popping up soon after launch and then diving down on the target. It only makes sense for them to use such tactics and weapons, we've seen how adaptable they are afterall.
 * Addendum: Who said they had FTL travel? For all we know they were travelling at STL speeds.
 * When the Marines are on the Sea Knights heading toward the FOB, you can overhear a radio conversation where an observer is reporting that the alien infantry are weaponry equivalent to heavy infantry weapons with firepower comparable to high explosive rounds.
 * Now, did anyone think the aliens' weapons seemed a little too advanced? They have man-portable missile launchers that can blow apart an Abrams, but no comparable armor of their own? And they have weapons that can go straight through body armor but seem to have no armor against our ammunition?
 * Having advanced weaponry doesn't necessarily mean they'll have advanced armor, especially considering that they're both an aquatic species and their biology renders them durable to most gunfire except when aimed at very precise points. Remember that the alien that went after Lenihan got shot about ten times at short range and was none the worse for wear. They may simply not believe body armor to be worth the weight and cost ratios.
 * I just find it strange that a species has developed weaponry for defeating incredibly sophisticated armor systems, yet appears to lack the armor that would need defeating. It just seems like they wouldn't necessarily be able to contend with our armor without having armor of their own to learn against.
 * ....which is why I said that "they may simply not believe body armor to be worth the weight and cost ratios" when their infantry can withstand dozens of gunshot wounds without much ill effect. Just because they have armor it doesn't mean they're going to employ it constantly; modern armies can theoretically outfit their entire armies with very heavy armor but it would impair their mobility to the point that infantry would be hobbled and ineffective. The aliens probably simply decided that their troops' ability to withstand small arms fire was sufficient as it was and focused their budgets/resources on drones and other areas. Especially considering a lot of the supplementary materials indicate that the alien soldiers are disposable drones.
 * We can also take into consideration we always prepare for yesterday's war (the aliens probably do the same). Imagine if the aliens had to previously face off against the weaponry they're now using against us, they'd more than likely realize a direct or even grazing shot would actually be less than favorable. If each shot is the equivalent of a small amount of high explosive being set off at the impact point then it's going to be the intense pressure and heat which would injure and kill, not the direct impact.
 * Also, in terms of mobility, armor basically automatically saps some of your mobility. We don't know what kind of technology they EXPECTED to be up against. For all we know they thought we we had just made it past the invention of the brick. If you were going up against a Stone Age - Bronze Age civilization, would you even bother with armor? Or would you rather have more mobility? Considering your weapons have SIGNIFICANTLY more range and rate of fire, I'd say you shouldn't wear much more than a stab vest.
 * IF the aliens army is composed of a ragtag militia, body armor would be saved for leaders and elites. Many militaries don't give their troops body armor due to expense or lack of resources. Many irregular forces make their own. The aliens have plenty of "manpower" so they figured that they could absorb the losses.
 * And as noted above, the aliens are pretty resistant to standard small arms. Only one small area of their body is vulnerable to small arms, unlike humans, whose entire torso and head and most of their arms and legs are vulnerable points that could result in fatal injuries. They don't need armor against human small arms, and no armor would really help them against heavier vehicle-mounted/crew-served weaponry.
 * Given the Marines had been told there would be an airstrike so they were just following their orders to get out of the zone, but the fact is that after the police station they saw the aliens already had solid air power. So why were the Marines so dang surprised when the airstrike didn't show up?
 * They were hoping an airstrike would show up, in spite of the enemy airpower. They weren't so much surprised as they were supremely disappointed. Also keep in mind that enemy airpower literally showed up an hour ago. They were expecting that the airpsace was still contested, as the notion of an enemy airforce establishing absolute aerial dominance in less than two hours is very unlikely.
 * Exactly. In the USAF, there are 3 bombers, B-1B Lancer, B-2A Spirit, and B-52H Stratofortress. Of the three, only one, the B-1, is super sonic capable. The other two are both subsonic, and the B-1B won't outrun one of those Drones which were giving F/A-18E/F Super Hornets (Mach 2 capable) a run for their money. On top of that, we don't have that many bombers in the inventory, and while a contested airspace isn't ideal, it would be better then watching what few bombers you have become scrap metal when your fighters are barely holding their own against the drones which are (atleast said to be) kicking the snot out of them. It would be just like sending B-17s over Germany during WW 2 with no escort, and anyone with a passing knowledge of the air war over Europe knows how that went.