[Endoperez]

They will starve. You'll have to give the insiders supply items (endless wine skin, cauldron of broth, summer sword, nature mages), or sites with supply bonus.

The trick is, essentially, that you can set units from different nations to start in the same province. Usually they fight on the first turn, but if there's a fort one side starts besieging the other. The fort will never be stormed, though, and there are independents and "special independents" who share the white flag.