Colony combat is simulated, using the relative strengths of both sides to achieve the result.
The attacker is limited to Marines brought in using troop transports (four units per). Dropping directly onto the surface of the planet, these troops fight to the bitter end. Their performance is determined by unlocked ground combat technologies (unlocked as tech achievements) and any innate racial traits. The defender has a more comfortable position, as on top of Marines stationed at marine barracks (if any), they can also employ armor units from armor barracks.
The alternative to conquering a planet is orbital bombardment. Firing indiscriminately, bombardment destroys population, colony structures, and any existing military units of the planet owner.
Any Marines left on the planet once it falls to the invader remain stationed as a permanent garrison, as transports cannot escape planetary gravity once deployed.
Battleoids are giant robot fighting vehicles with power and mobility far beyond that of conventional tanks. This technology allows your Armor Barracks to build and support Battleoids. Battleoids have a ground combat rating higher than tanks.