Enemy Exchange Program

In RTS games, at least the kind where the sides are significantly different from each other, it will often be possible to capture structures that produce enemy units and make enemy unit types for your own use. Pre-scripted single-player scenarios will sometimes make this mandatory or even restrict you to only using enemy units for the duration of the mission. This is an obvious way to add variety, but can easily come off as contrived story-wise.

A variation of this is having units that can take control of enemy units during battle.

Compare Monster Allies.


 * In Dune 2, the Ordos faction's "Deviator" tank could temporarily turn enemy units to Ordos control.
 * Capturing an enemy factory allowed you to build that enemy's "unique" unit (Atreides sonic tanks, Harkonnen Devastators, etc).
 * Weirdly, combining the above two examples did not work as you would expect. Some versions (and mods) allowed non-Ordos to capture an Ordos factory and produce their own Deviators, but no matter who owned the Deviator, it always turned its victims to Ordos.
 * Of course, taking over a unit for as short a period as you could didn't really do that much good...except against Devastators, which could be ordered to self-destruct.
 * The Command and Conquer games did this so frequently that it sometimes becomes a quick-victory tactic. The oldest and most common method was to send an Engineer into an enemy building. Newer games brought newer methods. Tiberian Sun introduced vehicle hijacking, Red Alert 2 had varying strengths of wireless mind control and as of Red Alert 3, there's bribing.
 * Certain scenarios can also trigger capture in ways that are not conventionally possible. For example, several times in the Tiberium series, Nod stole a GDI Ion Cannon by cracking into GDI's network infrastructure. Another type of scenario-triggered capture happens in the Red Alert: Counterstrike mission, "Sarin Gas: Down Under": an Allied Spy can hijack vehicles by infiltrating Soviet War Factories.
 * Quite possibly due to software limitations, unit quotes from Red Alert and beyond can be quite jarring if they are side-specific like, say, a GDI commander producing a Nod Militia squad whose one of many lines is "Down with GDI!" or an Allied commander's Ant whose response is "Vehicle reporting." Granted, the last one isn't so much Enemy Exchange, as much as it is a Game Mod.
 * Some games in the Age of Empires series have priests that can convert enemies to your side, even in the middle of the fight.
 * One of the scenarios starts you with a single priest, requiring you to convert any units that wander past in order to build an army - gets easier when villagers start showing up.
 * An extremely ridiculous example was the single-player campaign in the expansion pack for Age of Empires II, in which you played Aztecs defending against the Spanish invasion. In the final scenario, completing certain objectives would give you some Conquistador units (the Spanish special unit: mounted musketeers wearing plate armour) and a Turtle Ship (normally available only to the Korean faction).
 * It's possible to do this in War Craft II according to technical mechanics, but it's not a situation that ever occurs during campaign mode. In custom scenarios, it's possible to confuse the game's soundbytes by abusing the mechanics, resulting in the Command and Conquer situation where human footman would start grunting like orcs.
 * The expansion for Warcraft II, however, had such scenarios in each campaign (Alliance and Horde).
 * In Warcraft III, the Undead Banshee unit can possess a single enemy unit, sacrificing itself to give you control of it. The Death Knight hero can temporarily raise up to 6 dead units regardless of former allegiance to fight for him, although without any special abilities or spells that the unit might normally have.
 * It should be stated that, although normally posessing a builder with a Banshee locks out their building capabilities, the High Elf worker which appears in the same Campaign mission where the Banshees are introduced does not, meaning you can then begin constructing their structures and, by effect, units.
 * Wait, it's supposed to lock their building capabilities? I distinctly remember building a small group of Human units when playing a LAN game just for laughs...
 * The Expansion added the neutral Dark Ranger hero, who can permanently take control over a single enemy unit, abilities and all, every few minutes.
 * Blood Elf Spell Breakers can convert summoned units.
 * In Starcraft, the Protoss Dark Archon has a Mind Control ability that can capture enemy units and convert them to your side. If it's used on an enemy worker unit, you can then use it to build that race's units - and since the three races all have separate Arbitrary Head Count Limits, it is then possible to raise up 200 units of each race in your attack force.
 * Additionally, the campaign editor lets you set any unit to "rescuable"--including units whose race doesn't match the player's (or you can cut to the chase and implement triggers that give players units of other races, either by spawning in additional units or changing control of existing ones). The first game, where Dark Archons weren't available, makes use of this in the official campaign.
 * The Zerg get an aversion though: an infested command center is acquired by beating an enemy command center to the red, then getting a Queen unit to the building before it explodes. For your trouble you get an entirely different building that makes Infested Terrans, a unique unit which is also far different from what a Command Center usually produces.
 * In Star Craft 2 there is an achievement called Zerglot. While playing as Zerg against a Protoss opponent, use an Infestor unit's mental parasite ability to take temporary control of an enemy probe, which is the Protoss worker unit. While the probe is still under your control, build a Nexus, the Protoss Command Building. Then continue building Protoss buildings with probes built at the Nexus until you can build a Zealot, and then build it. This earns the Zerglot achievement. It is possible to do this in any game mode, but playing against the AI makes it much easier. It is also possible to do this with Terran units by capturing an SCV, but it will either have to be captured again by different Infestors, or multiple ones will have to be captured sequentially to build a Command Center.
 * In Total Annihilation both commander units can capture enemy units, including construction units. Core has a resurrection unit that can raise enemy units. In the sequel, Total Annihilation Kingdoms Elsin, the Aramonian monarch, can resurrect enemy units and Zhon's Harpy and Taros' Mind Mage can capture enemy units.
 * In the Spiritual Successor, Supreme Commander, any unit that can build or repair can capture enemy units and structures, though is generally easier to just salvage them. Becomes extra fun when you manage to steal an enemy's engineer when they're a different faction to you. Since a single engineer with some resources can construct everything needed to make an entire army, one can then build practically ANYTHING normally reserved for your enemies with a bit of time.
 * Supreme Commander 2 has a mission where an an AI has gone insane and builds engineers and turrets nonstop. Staying in control of your base is surprisingly hard until you get the hang of it.
 * The same applies in the Dungeon Keeper series, where enemies can be converted in the torture chamber, but neither they nor your creatures will enjoy each other's company, and will likely require separate dungeon areas.
 * They each have a set enemy. Whilst they won't enjoy being in the company of most of your units they can tolerate it, if you keep them well fed and amused. However, if you put a guard next to a dark mistress, the lady who tortured him, expect conflict.
 * Also used in the single-player campaign in World in Conflict. There are several missions in which you can take control of abandoned enemy units by repairing them, though this isn't possible in normal gameplay.
 * There is also a ludicrous mission in which Soviet special forces have seized control of the Statue of Liberty, Ellis island and Governor's island. They use a large number of captured US vehicles against the player. One character lampshades this by remarking that the number of vehicles available to them is very high, and another replies that the post was oversupplied and undermanned. It is not explained why these islands had any heavy vehicles on them at all.
 * Possibly the oldest game that does this is Populous II, which allows you to place "baptism fonts" with water magic, which convert troops from one side to the other (and back). The prequel, Populous the Beginning (the earlier Populous games are god games, and this one is a straight RTS where the plot involves becoming a god) has the enemy-converting priest as one of the basic troops.
 * In the Heroes of Might and Magic series, you can start training new troops to reinforce your army the moment you conquer an enemy city, so even if you have an army of do-gooder forest elves, you can still add Demonic Invaders or The Undead without anyone trying to gut the enemy they've hated for generations.
 * To be fair, mixing them under the command of one hero generally leads to morale penalties.
 * The fifth game in the series hangs a lampshade on this. There's a scenario where the demonic commander captures some forest elf cities and builds an army of elven troops. He then has a limited time in which to capture a demonic stronghold before the massive and predictable desertion rate leaves him with no forces at all. The same character later turns the other heel and finds that demonic forces will thereafter destroy their own cities rather than let them fall into his hands.
 * Age of Wonders is another TBS fantasy game that allows you to mix units of any faction once you capture a city of theirs, but it generally causes issues due to morale penalties if you try to mix different alignments. Units might abandon your cause, and cities might rebel unless a sufficent force is garrisoned in it. However, the game also gives you the option to convert cities to allied races or just pillage and burn them.
 * Globulation is an extreme example, as converting all enemy "globes" is a more standard way of winning than killing them.
 * The tabletop RPG Dungeons and Dragons allows clerics (well, evil clerics) to use their Turn Undead ability to control undead creatures and have them fight for the party as expendable minions. This doesn't work any more as of fourth edition.
 * Seven Kingdoms allows you to easily reproduce each race's buildings and units, with the exceptions of the powerful Seat of Power, which can only be built if you have the respective scroll (and each human nation starts with only one). However, human players can't build any of the demonic Frythans structures even if they control some of their units, but it works the other way around. (Getting a large enough number to operate the structures is another issue entirely, as well as keeping them from rebelling).
 * Homeworld has the Salvage Corvette that can pick up small enemy craft or latch onto larger ships and drag them back to your Mothership, where presumably the enemy crew is subdued (or worse). The enemy ship is then re-launched with a friendly crew, allowing you to control it. The Game Breaker is that this allows you to exceed the Arbitrary Headcount Limit, potentially giving you a fleet far larger than you could possibly build on your own and making later levels trivial.
 * Homeworld 2 also allows for capturing with the Hiigaran Marine Frigate and its Vaygr equivalent. The frigate flies up to an enemy vessel and launches a boarding party to take over the ship. However, it's a lot harder to do this than with Salvage Corvettes in the first game, and the game's Dynamic Difficulty makes larger fleets less useful.
 * Played straight and averted in the Warlords series. In the first two games, once you occupy an enemy city (without pillaging or sacking it) you can produce any units that the enemy could in that city, including special units that are specific to the enemy side and otherwise unavailable to you. In the third game captured cities cannot produce those special units unless your side is already capable of doing so.
 * Averted in Warlords Battlecry 2, whereas the hero can convert enemy buildings, but the buildings can not be used to build units. They add to your army limit though.
 * Warhammer 40000: Dawn of War: Dark Crusade introduced the Necron Lord Destroyer, which could "possess" enemy vehicles and convert them to your side. When these vehicles are destroyed, the Lord Destroyer pops out damaged but "alive". Seeing as you could only build a maximum of two of these, though, using this is situational.
 * Halo Wars gives the UNSC access to SPARTAN Super Soldiers, who have various abilities. One of these includes the power to hijack enemy vehicles, just like they can in the Halo FPS games.
 * Averted in Star Trek Armada: to capture an enemy ship, it's shields must be disabled and then boarding parties must be beamed onboard, taken from the capturing ship's crew compliment. The boarding crew will invariably take casualties as well, and ships have a minimum crew requirment before they will operate at full efficiency. The Borg, however, play it straight by holding ships in a tractor beam and transporting the enemy crew off to be assimilated, though they can also capture ships the 'standard' way.
 * Also played somewhat straight in that capturing an enemy construction ship allows you to access the entire tech tree and build a complete fleet of ships of the construction ship's original race. They will, however, be crewed by members of your own race, often with hilarious results such as a Romulan-manned Borg cube responding with, "Warbird reporting..." when selected because the sound bites for a given unit type (in this case "battleship") are simply transfered straight across (in this case from the Romulan Warbird to the Romulan-manned cube) to all "battleship" type units manned by your race without regard to whether they are fitting or not... Also, by the way, only the Borg's Assimilator unit transports the enemy crew off their ship to be assimilated (where they are either added to the Assimilator's crew or added to your general crew pool if the Assimilator has a full compliment) and it does not actually hold the enemy ship in place. The Borg cube's holding beam does hold enemy ships in place, but it just does exactly what the transporter does except on a larger scale (more Drones are transfered per second than with the transporter) and able to do it through enemy shields... Incidentally, capturing an Assimilator as another race does not remove its special ability. This has the hilarious side-effect of having, say, a Federation-manned Assimilator "assimilating" enemy crew, including the Borg player you stole it from in the first place.
 * Warzone 2100 had the NEXUS link turret, which infected enemy units with a virus that made them fight for your side. Somewhat subverted that in single player, it's mostly the enemy NEXUS faction using it on you, and by the time you research the link turret yourself, NEXUS has the Resistance Circuits tech (also obtainable by the player) that makes the link turret ineffective.
 * Real Life: During WW 2, this was an integral part of German war strategy. On field level, German forces often captured and used enemy tanks, cars, and cannons (even horses were used). But it was also extended to larger scheme with strategical resources: Germany produced most of light tanks in Chechoslovakia, and heavy tanks were produced in France, after both lands were captured. Also additional planes were constructed in the Netherlands' existing factories, not to mention Austrian ones, which supplied antitank cannons. This explains how Germany was able to fight successfully against Allies while having much less resources.
 * The Soviets did the same--The 5th Guards Tank Brigade even got their first T-34/85 by capturing it from the Germans who had previously captured it from some other Soviet unit.
 * And earlier in history, the Ottoman Turks used janissaries, who were originally forces composed of captured citizens trained to fight for the Ottoman side.
 * The Nazis also recruited a sizable amount of soldiers from the occupied territories. Volunteers, that is. If they hadn't been so blindingly stupid (and, you know, racist), they would have had a much easier time in the Soviet Union, where they were initially welcomed by the populace. Though they were tot necessarily volunteers. Stories exist where the 'volunteers' turned side at first sight of Allies, and about one chinese who was captured during war, taken into red army, transferred to eastern front during WWII (western from POV of soviets), captured by Nazis and transferred to Normandy where he was captured by Allies.
 * The Waffen-SS was the only branch of the Wehrmacht which recruited outside Greater Germany. A great deal - perhaps a full half - of the SS was non-German when the war ended. The last troops defending the Reichstag were from a French SS unit.
 * Of course, while good for production, it led to the logistics becoming a quagmire, since each gun would only work with its own ammunition, and likewise, each engine could use only its particular parts.
 * Star Wars: Galactic Battlegrounds has Jedi and Sith Knights and Masters (well, Sith Lords anyway) who can convert units and buildings. Of course, because 75% of the differences between the sides' units are cosmetic, this is only really useful for stealing unique or expensive units, protecting the Jedi in question, or taking advantage of racial special abilities such as the Gungan frigate's ability to submerge.
 * The Elder Scrolls Oblivion lets you summon various daedra and other such things that would normally be enemies. But they turn on you if you attack them enough.
 * In also features limited duration command spells that get enemies to fight for you.
 * Space Empires IV allows a player to capture enemy ships, steal enemy blueprints, and reverse-engineer enemy technology. So it's downright easy to get someone else's ships--though only capturing them outright will produce ships that follow the enemy design.
 * Machines Wired for War features allow you to take over enemy buildings as well as steal unit design plans, also the Judas warlord that can convert enemy units to your side.
 * Done in Aztec Wars. In fact, constructing some of your own units requires a building that can be only built by an enemy nation.
 * In Operation Flashpoint, you can literally use any vehicle that isn't locked, so this can and does happen. One mission in the Resistance campaign involves stealing several enemy tanks.
 * Similary, none of tanks and helicopters in the Battlefield games spawn locked, so although each side has their own vehicles, you need to rely on the color of the player's name above the vehicles to determine which vehicles to shoot at.
 * In Rise of Nations, the Spy is capable of converting enemy units to the other side using the ability Bribe. However, the normally invisible Spy is revealed after the ability and is partially revealed during the casting (usually during this time they are insta-killed by the Counter Intelligence ability). Apparently, the vanilla Rise Of Nation had the Russians produce spies that don't reveal after using Bribe, so it was kind of a Game Breaker.
 * Boarding and capturing enemy ships in Master of Orion can yield valuable technology when you scrap them if they have systems or weapons you haven't researched.
 * An interestingly version of this occurs in Achron: The CESO Heavy Tank has the ability to infect enemy units and structures with nanites. The subverted units still retain the colour and the ability to take orders from their original faction... but you can give them orders whenever you want to too. Instant spies and/or traitors!
 * Sword of the Stars have boarding pods that can be used to take control of the guns of enemy ships, turning them into immobile weapons platforms for your side (presumably, the bridge crew disable the ship's central control before being overrun). Ships 'captured' in this way are destroyed at end of combat.
 * While there a few provinces in Medieval Total War that specialize in a unit and will produce them for anyone, most troop types are tied to the factions. So if your English crusader army captures Egypt the locals suddenly figure out how to grow yew trees in the desert but forget how to herd camels.
 * In Mount and Blade, most units are recruited from villages. These villagers are then trained into the specific soldier class of the kingdom they belong to. However, their birth does not change, even if the country that rules them does. Therefore, it is possible to hire recruits of an enemy nation's soldier type once you conquer parts of their kingdom. There are also a few more complicated ways, such as capturing them in battle, crushing their morale, then offering them a second chance by joining you.
 * In Cossacks: European War, you can capture an enemy peasant (or artillery piece, or civilian building, or military building under special circumstances) when your military units are nearby and the enemy's aren't. Enemy peasants however, retain their home nationality, allowing you to build up their tech tree as well as your own. This is less useful than it first appears though, since not one single piece of research you've done crosses over, requiring you to build the new civilisation from the ground up.
 * inCompany of Heroes, many weapons like mortars and heavy machine guns require crews to operate. If your troops manage to kill the crew of a weapon without destroying it, they can pick it up and turn it on its original owners.
 * In the Hegemony Series, every city has a culture and can only recruit units of that culture. They'll cost extra, but you'll have enemy units.