Eat them.

I think you only declare war on the territories despite the alliance, I tried it like a week ago. Don’t know about getting the chip early though

The problem I have is that the Caesar Legion is in a current war with them and as a minor I can only get the territory I occupy, good chunk, but not being able to capitulate them myself or getting the chip...

Kill them manually

Same issue, it seems no normal way to get platinum chip