[Endoperez]

All movement orders in friendly territory happen first. All other movement orders happen after that. That means that the enemy has already left when your troops get there.

All magic happens before movement, so teleportation etc can be used to catch enemy armies.
Hiding/sneaking happens before movement, so you can't fight a stealthy army unless it attacks.

I'm not quite sure about this, but I think that moving army away from a besieged province (which still owned by the defender) happens after the defender has moved his forces there, so there'd be a fight.