The AI likes to hit and run. When you start as Scipii and chuck all your forces at the Greek city over on Scily, if you leave only one unit to guard Messana, the Carthaginian elephant stack will come in along the coast into your lands. When it happened it didnt have enough MP to move all the way. So i moved a few Hastatii, nothing much, back into messana to deter the com from finishing his raid. That sorted them out. I assume an ally might do this as well. After all, it only takes a single act of aggession to start a war.