Just caught up =)
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The province must be occupied by someone other than rebels, must have a garrison of 1000+, cannot currently be under siege, and the aforementioned WE scores. I think there's also a War Score requirement, but I'm not at home to look. It's in morerandomevents.txt - search for war_exhaustion=15 and you'll eventually find it in there. There are no culture or religion requirements (although religion is a modifier to the MTTH).