Fully establishing an outpost or a colony could require a supply line to one of your core systems, or simply that the system is controlled by your ships, or at least free of enemy ships.Until then, they would still be considered neutral territory for cold war purposes, meaning you could attack them or perhaps even build a competing outpost in the same system (and then pretend you were there first ). The legitimacy of an outpost could take some time to establish, instead of instantly including the system into a regime's core worlds.However once one of the two parties has attacked or been caught red-handed ambushing or sabotaging, both can engage in them.
A peaceful policy would only allow covert actions, initially. Your first contact policy would dictate if you're allowed to engage in hostile cold war actions first.In the case of an AI target it would tend to reply in kind, too, probably escalating a little. Of course, both for this kind of attack and the aforementioned sabotage activity if you get discovered or suspected, your relations with the target race and your overall reputation would suffer. We should also be able to mount an direct, undisguised attack on an outpost or ship prancing through our own territory or through neutral space without declaring war.Perhaps this could be a mechanic linked to espionage and envoys. Overall, when a player or AI just spams explorers and builders far from their home, it feels weird that I cannot arrange "unfortunate accidents" for those, ideally trying to disguise the killings as pirate activity or exploration gone awry.