5. DEFENCE AND CONQUEST
Defensive installations on planet are auto built at a rate determined by your Defence Budget (Finances/Budget tab). Those installations will fire upon any enemy squadrons that enter the orbit of the planet.

Imperial infantry is garrisoned on planets and built similarly to defensive installations. They can’t be moved and will fight till the end to defend the imperial planet. You can grant a “defence subsidy” (Planet/Administration tab) to strengthen the defensive installations and troops on the planet, but note that all subsidies expire at the end of era.

Stormtroopers are elite mobile infantry used for conquering planets. Once your squadrons take over control of the planet orbit stormtroopers will be automatically dropped to the ground (it might take several turns to deploy them). Note that the empire has limited number of stormtroopers so conquering several planets at once might be not advisable.

Invasion starts after your squadrons take control of planet’s orbit. Your squadrons start bombarding the planet and stormtropers start to be dropped. There is an invasion progress meter, once it reaches 100% the Empire takes over the planet. If at any moment you lose control of the orbit (by moving squadrons away for example) all invasion progress will be lost and you will need to start from scratch. After the planet is conquered stormtroopers will be recalled (it can take several turns).
Since the enemy need to control planet orbit to start an invasion do not bother with any remaining enemy ground troops, those are in the process of retreating.