Space combat occurs at a planet between two or more empires, or between an empire and an independent planet. During space combat, a fleet in orbit attacks (or is attacked by) another fleet and/or the planetary defenses and ships stationed on the planet. Fleets or planets begin an attack the instant that they receive the attack order; the same is true for retreat orders. Fleets arriving at a planet where combat is already occurring will join the battle immediately.

Space combat can be initiated from either the map screen or the combat visualizer screen.

During an invasion, all fleets orbiting a planet will have their status given as “invading”, even if they belong to empires that are not participating in the battle. This is a known bug.

Fleets are divided up into wings for the purpose of player control. Each wing contains a single unit type. Units in a wing move and attack as a group. Individual wings can be seen and given orders in the combat visualizer screen.

Wings move between different orbital heights to engage targets; planetary defenses do not change their height. Ships with long attack ranges may be able to orbit out of range of some defenses and can engage their targets for longer periods of time per orbit. Units, weapons fire, point defense interceptions and area of effect explosions can all be seen on the combat visualization screen.

All weapons and point defense systems fire at the same rate. The period of time between weapon shots is a “combat round”. Direct attacks hit their target in the same combat round that they are initiated. Missile attacks may not hit until several rounds later, depending on how far away the target is and how fast it is moving relative to the attacker. Ship destruction is calculated once per combat round.

Multiple empires can participate in a single battle. For a fleet from a third empire to join ongoing combat, the third empire's player must picks a fleet or planet to attack. All subsequent combat during the battle will be against the targeted empire and will ignore the other empire. However, if any player manually orders wings to attack wings from both other sides in the combat visualizer, they will continue the battle against both other players.

Jumpmissile attacks are nearly instantaneous. A simple attack between small fleets can resolve within a single watch. Invasions involving big fleets can take significantly longer. Battles do not “time out”.

Space combat ends if the attacking player orders a retreat or if either side takes a certain % of space forces as casualties. However, either participant is free to resume combat if they wish. If the losing side controls the planet and the attacker is performing an invasion, combat continues; the attacker's transports will start to land and ground combat will begin.)

The “attack” and “invasion” orders are functionally the same, except that during invasions transports will attempt to land ground units and begin ground combat after space combat has mostly finished. Transports can be manually forced to land at any time by selecting them in the combat visualizer and ordering them to land; this can turn an attack into an invasion.

Ground units are the only units that take part in the ground combat stage of an invasion; ships do not participate. Ground combat is more straightforward than space combat; it does not have a designated visualization, and the stronger force usually wins. More advanced ground unit types can win against slightly stronger forces of less advanced types. If the invading force wins, it conquers the planet.

If one force of ground units is significantly weaker or smaller than the other force at any point during combat, it gets captured by the other side. Captured units join your forces, so you can actually end a battle with more units than you started with.

Ships and planetary defenses can be captured if they are present at or in orbit of an allied planet when it is conquered. This is most commonly seen if transports were forced to land before enemy defenses were destroyed, or if a reinforcement fleet arrives at a planet after transports have begun to land.