After bombing a base from orbit, you may need to land troops to take control of what's left. Alternatively, you may want to try and take a base intact, in which case infantry and supporting forces are the only option. An invasion force can be made up of six different types of units, each with their own advantages and disadvantages.

Icon	Unit	Cost	MASS	Description
	Riflemen	5	3	Basic infantry units, used for holding ground and entering bases. Good against: Engineers. Bad against: APCs, Tanks, Gunships, Artillery
	Engineer	5	3	Specialist infantry equipped with heavy weapons and demolitions. Good against: Tanks, Artillery. Bad against: Riflemen
	APCs	5	4	Used to carry infantry into battle, and also equipped with basic AA defences. Good against: Gunships. Bad against: Tanks
	Tanks	10	2	Armoured fighting vehicles designed to go up against vehicles and emplacements. Good against: APCs, Artillery. Bad against: Gunships
	Gunships	15	2	VTOL or other airborne forces capable of rapid movement and first strike against vehicles, but not so good against infantry which can easily take cover. Good against: APCs, Tanks, Artillery. Bad against: Riflemen, Enginneers
	Artillery	15	2	Heavy mobile artillery, capable of hitting targets at long range. Good against: Riflemen, Engineers, APCs. Bad against: Gunships

All these need to be carried down to the surface. It is assumed that they can all operate in either an atmosphere or vacuum environment, but can't operate in the depths of space.