Right, except I couldn't think of any scenarios where this would be true when you're in your own realm. I think I have now though, so never mind.
If Bob of Reprehensiblistan is sitting in one of your regions, and you move there with murderous settings, you will engage.
Whoever arrives first controls the battlefield, in my experience. Unless you arrive in the region he was sitting in without starting a fight, then odds are you'll gain control of the battlefield.