Target Earth

I think these gentlemen may wish to harm me.

Rex: The Earth must not be a battleground again! We defend her.
Rance: And we shall never reconcile with you!

Developed in Japan in 1990 for the Sega Genesis (and known as Assault Suits Leynos there), Target Earth is a side-scrolling shooter and perhaps one of the earliest Humongous Mecha video games. It is also Nintendo Hard, and comes with a well-developed plot.

An Opening Scroll (briefly) establishes the background: Earth's outcasts have returned with a vengeance and attacked the planet in a vicious assault. The Earth Defense League tries its best to hold, but later levels reveal that Earth has been at least partially overrun. The end of the intro and the beginning of the game focus on Ganymede, one of Jupiter's moons.

Rex suits up in his special Assault Suit just as the base comes under attack, and so begins a brutal campaign to defend Earth from her assailants. Rex faces down hordes of Mecha-Mooks, Heavily Armored Mecha-Mooks, Elite Mecha-Mooks, and enemy warships, backed only by his Redshirt Army of bronze-colored Mechs and the far more useful officers and allies who provide intel via comm transmissions. Through the course of his battles, Rex learns more about the enemy that has attacked them seemingly without provocation, and begins to doubt the righteousness of the war.

Tropes used in Target Earth include:

Best Served Cold: The Chron survivors built up their forces for 100 years before attacking Earth.
Bittersweet Ending
Critical Existence Failure: Both played straight and averted. All mooks, and the player, for that matter, can take their blows with no signs of damage and no decreased functionality. Averted with the warships, however. As you blast away their HP, they start losing parts of the ship, including turrets, making them easier to kill the closer they are to death.
Cyborg
The Dragon: Rance Culzus.
Emergency Weapon: The gatling gun that serves as your primary weapon never runs out of ammo (though it has to be reloaded every 20 shots). However, you can remove it from your arsenal during the weapon selection segment prior to each level, which would probably make most of the levels unwinnable.
Graying Morality: As the plot progresses, Rex begins to see that the righteous fight to protect Earth isn't as clear cut as it had seemed. In the first few levels, he's a gung-ho pilot ready to win the war. By the time he kills The Dragon, all he has to say is "Another good man dead".
Humongous Mecha: Very few soldier-level Mooks and Red Shirts aren't Humongous Mechas.
Informed Equipment: The only addition that actually appears on the player's mech is the shield--weapons all look the same and extra armor makes no changes to appearance.
Last Chance Hit Point: The player character's health.
Killed Mid-Sentence: Rance, as he is defiantly telling you that he can still fight.
Man in the Machine/Cybernetics Eat Your Soul: The survivors of the OSE program became part machine in order to survive by constructing and utilizing the Chron Cyborg Creation system. What exactly that is and what it involves isn't very well explained; Rance, for instance, appears human until after his battle at one of the Chron systems, and then is sporting noticeable cybernetics; however, he says that they used the Chron to survive 100 years ago and that "now the Chron keeps us alive". It's possible that they all became cyborgs in order to survive the length of time needed to get revenge, but if they are injured in battle, then they have to have even more implants added. Since the Chron isn't clarified, the reason the cyborgs are so vindictive could be part of Cybernetics Eat Your Soul.
Mini-Mook: Almost every enemy you face is either a Humongous Mecha or some sort of oddly shaped vehicle. Towards the end of the game (or right at the beginning, depending on what difficulty you're on), dropships sometimes fly by and deploy actual footsoldiers. They're full sized humans, but to you, they may as well be ants.
Nintendo Hard: With a vengeance. Your health respawns on its own, so long as you aren't constantly being damaged. Guess what? The enemies never stop coming.
No Campaign for the Wicked: Somewhat subverted. You start off as "the good guys", but it slowly becomes apparent that there's more to this conflict than first appeared.
One-Hit Polykill: Only one weapon: the laser.
Poor Communication Kills: The reason for the entire war.

Rance: We are the survivors of the Outer Space Expeditionary program. We left the Earth 100 years ago with all of man's hope. But we had trouble at the edge of the solar system. We radioed Earth for help but there was silence. We have been forsaken and are here to take our revenge!
Rex: But 100 years ago there was a world war. We had to rebuild from the ground up. There was no way the Earth could have communicated with you! Your expedition held hope for us all. We never gave up that hope. Now look what a failure to communicate has caused. War.

Real Is Brown
Regenerating Health: Too bad the mooks won't leave you alone long enough to take advantage of this.
Story-Driven Invulnerability: Rance's first appearance has him shoot down your friendly escort warship, and he is completely immune to damage (but doesn't attack you--apparently, you're beneath his notice at the time).
Suicidal Overconfidence: See Mini-Mooks above.
Too Dumb to Live: Literally. The enemy bombers are incapable of adjusting their altitude, leading to a great deal of laughter when they plow into the sides of mountains.
Video Game Caring Potential: It's hard not to feel Papa Wolf-y towards your allies, especially the mechs that are half your size.
- Particularly in the second mission, where you have to board a shuttle evacuating the base. As soon as you do, it closes its hatch and takes off, but the longer you stay, the more redshirts can make it aboard.
What a Senseless Waste of Human Life: Rex laments Rance's death, and the entire, unnecessary war.