You can RGA any planet, starbase or not, foreign or own. If you use RGA you normaly set no primary enemy (PE) because you want to RGA _instead_ of fighting.
It is the preferred way for the Rebels to destroy starbases by RGA'ing for several turns. RGA lowers the happiness of the settlers and finally they will start fighting and killing each other. Once the last settler has died a starbase in orbit will vanish.
Why RGA instead of attacking with a Rush? The Rush often loses against a fully fortified base in combat and in real games starships are your most valuable resource. You can't replace them as easily as in this training game. This is why RGA is one of the most powerful ability in Planets - it allows to destroy enemy starbases without losing a ship.
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