Official rule text
This ability can be used even if no ships were moved during the 78 SPACE COMBAT 77.4 “Move Ships” step. After resolving the “Space Cannon Offense” step of a tactical 77.5 STEP 2—ASSIGN HITS: The player whose units have been action, if two players have ships in the active system, those players targeted by “Space Cannon” must choose and destroy one of must resolve a space combat. their ships in the active system for each hit result produced against their units. 78.1 If the active player is the only player with ships in the system, they skip the “Space Combat” step of the tactical action and a Players other than the active player must target the active proceeds to the “Invasion” step. player’s units. 78.2 If an ability occurs “before combat,” it occurs immediately b If the active player is using the “Space Cannon” ability of before the “Anti-Fighter Barrage” step. their units, they choose a player who has ships in the active system. That player must choose and destroy one of their a During the first round of a combat, “start of combat” and ships in the active system for each hit the space cannon roll “start of combat round” effects occur during the same produced. timing window.