Manual:Game Mechanics/Planetary Invasions

Once all of a world's defenses and all ships in that system that are defending have been eliminated, you may attempt to occupy a world. You must initiate a planetary attack, which takes 0.1 turn. The defender's strength is equal to their security rank +2 for every 5 infrastructure on the world. The attacker's strength is equal to their total fleet size plus their security value (note that if a ship is at less than 1/2 base boarding strength, it cannot aid in planetary attacks). If the attacker has more strength then the defender, they spend 2 full turns overtaking the world. If the attacker has at least twice the strength of the defender, it only takes 1 full turn to occupy. If the attacker has at least 3 times the strength of the defender, they overtake the world in 0.5 turn. Occupied planets cease all construction and production, and do not gain infrastructure.

After invading a world, the attacker can choose to either maintain the occupation, pillage the world, or conquer it. If you pillage a world, you may load up to twice the world's resources (ore/gas) into whatever supply modules the fleet has. When you pillage a world, each structure on that world is reduced by 1 level. You may not pillage that world again until the structures have been rebuilt and the occupation lifted.

Conquering a world takes a period of time equal to that of the original occupation (using the original defense and attack strengths). After such a time, you gain control of that world. Note that this counts against your normal limit of controlled worlds. The world skips resource and infrastructure gain on its following turn, but afterwards you may use the world's resources and surviving structures as though they were your own. Note that conquering a world gives your race a good chance to unlock the mysteries of some of the aliens' technology.

When a defender is in danger of having one of their worlds occupied, they may choose to drop up to 4 levels of structures on that world in an effort to prevent the enemy from taking control of them.