If you don't own the planet, your troops invade the regular way during combat.
If you currently own the planet, you can drop cargo.
BTW, that's not the reason combat goes on for years in gritecon
The main reason is the 1 round of combat per month, and the 1 attack/50 hp infantry.
Because of that ratio, it takes a while for your tanks to mop up militia and get the planet under control, but in the meantime you can be happy its not producing resources for the enemy.
PS:
Bombardment from space may be slower than a ground assault in this mod, since a planet full of infantry will have hundreds of thousands of hitpoints
.
Ships will run out of supplies pretty fast during a bombardment, while tanks just keep rolling and stomping.