The AI in this game does some... odd things.
Playing as the Danes, I've expanded all across East Europe, stopping at the incredibly valuable port at Khazar. I've got Longboats trading from Lithuania all the way to Khazar, getting most of the ports in between save for the north coast of Africa. Money is pouring in.
Along the way, Crimea falls to a neutral faction. I ready my army to take it, but the Hungarians beat me to the punch. I start putting troops on the border, and they do the same. Numberwise, we're about even, but I attack. I push them back a few provinces(including Crimea), not entirely aware of the size of their nation.
Eventually, the Pope demands I stop attacking them. I comply. Apparently, this does not apply to the other side, since they launched a hilariously abortive attack on me with their king. In the meantime, the remains of their territory are split in two with a civil war, which might have been started when I put an inquisitor there.
Despite the fact that the province on the southwestern side has about 1800 men, their king(on the NE side) with less than 100, attacks the rebels. As far as I can tell, he's old and has no heirs, though the rebel leader claims royal blood.
Next turn, I see the king's band back where he started, reduced in size, and with about 25 men being returned as ransom.