Invasion

Official rule text

49.3 STEP 3—SPACE CANNON DEFENSE: If the active player commits any ground forces to a planet that contains units that have the “Space Cannon” ability, those “Space Cannon” abilities can be used against the committed ground forces. a If the active player committed ground forces to more than one planet that contained units with a “Space Cannon” ability, the active player chooses the order in which those “Space Cannon” abilities are resolved.