There are a wide range of diplomatic negotiations.
Players can also threaten each other, declare war and arrange cease-fires.
Each AI player remembers others' actions, both positive and negative, and will be unwilling to form alliances with a player who has broken previous treaties with it.
Hostile actions do not automatically cause war. Clashes are even expected at the opening of the game, when all sides are sending probes out into the unknown. On the other extreme, a ground assault must be knowingly targeted at an inhabited planet, and is a massive provocation.
Colonies can be bombed from space, or taken in ground invasions. Ground invasions can be conducted through enemy defenses. Present enemy ships or missile bases will fire on the approaching transports, possibly destroying some or all of them. The invasion itself is fully automatic. Results depend on numbers, technology and (if one of the races involved is Bulrathi) racial ground combat bonus.
Invasion is expensive. There are no special soldier units; colonial population itself is sent to fight, exterminate the existing inhabitants, and form a new planetary population. The production capacity of any remaining factories can be gleaned, and plundering of technologies if enough factories survived the attack. Controlling a new system extends the range of the invader's ships.