Ground combat is a special sort of combat which occurs when a starship drops colonists onto an planet not controlled by the starship's owner. Please note that, if multiple starships drop colonists on an unoccupied planet (or one occupied by a third party) in the same turn, the drops take place in ascending order according to Ship ID number. In that event, each succeeding surviving assault wave is considered the attacker when it is being dropped and the defender when it is being dropped upon.Certain races have a default advantage in ground combat, as follows:
Every defense post on a planet adds a defense factor of 0.05 to defending colonists on that planet. (20 defense posts increases a planet's defense factor by 1.) This is then multiplied by racial modifiers. This means 1 defending Federation clan on a planet with 60 Defense Posts would kill 4 attacking Rebel clans, or 20 attacking Rebel clans if the Federation has the Fortress advantage active.
If a ground attack is successful, all planetary structures (including defense posts) remain intact, and any orbiting starbase changes hands as well. When a planet is taken with ground combat, the Friendly Code of that planet is not changed. If the planet has a starbase, the Primary Orders of that starbase are reset to None and any queued build is canceled, the components are stored and the unused hull is destroyed. In addition, all fighters, torpedoes and starship components (beams, tubes, and engines) will survive, but any hulls at the starbase will be destroyed.
If at any point after ground combat begins it should happen that the planet is reduced to 0 Colonists, any orbiting starbase will be destroyed.
An Imperial Assault is a special case of ground attack that bypasses defense posts, destroying them in the process; 10 or more dropped clans from a Super Star Destroyer will automatically defeat all defending colonists. This takes place during the same Host phase as all other ground attack, so depending on Ship ID numbers, it's entirely possible that an Imperial Assault force will be attacked and wiped out in turn.
NOTE: In games featuring the Horwasp Plague, combat of that nature occurs later in the Host Order. For details, refer to the main Horwasp page and those beneath it.