We already have something as you say. For some rebel cities, attacking them will cause their parent faction to declare war on you. For example, the polis of Sinope will ask KH for help if the player tries to attack it. Firstly you get a warning from your advisor, if you continue to proceed then KH will declare war. We cannot make the city actually become part of KH (or any other faction), but this is a better option anyway, otherwise KH would start gaining cities everywhere, and their alliance was never that strong.

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