If 2 factions attack, what determines...

Ok I bribed an army of rebels. The same turn (The turn they turned all Irish) The Vikings sent a unit in, I had more men but they got to defend...why? The Rebels had it so I should have defended?

I've had this happen. As the Irish I bribed Rheget(?) and the same turn the came over to me the Mercians invaded but I was the attacker.

It appears to be treated the same way as when 2 armies move into a province at the same time, you are always the attacker. It is odd since in the case of bribing rebels the rebels are already in the province.

However, think about what would happen if there were two armies in a province and you bribed one but not the other. Your bribed army would be the attacker, so that is probably why it works this way.