What determines battle order?

[Norfleet]

If the province belongs to a third party, a randomly selected person fights that third party, and then defends against the other party.

This changes if the province in question is/was owned by one of the two parties: If you are moving to a friendly province, you will always move first. If you are moving to a province which was formerly friendly, but has now been occupied by the enemy, and the enemy is moving out of that province into another one of your provinces, you will move after he does, every time.

There's other really complex rules for what happens if more than one army attempts to move into a hostile province, perhaps while a hostile army from a province, either another, or the target attempts to move into a province occupied by one of the moving armies. In such cases, the game selects the most disadvantageous outcome for the player attempting the most elaborate manuever, and picks that.