Who ever is the defender when the pact is broken gets +1 defense to all their units on the first round of combat, in combats launched by that opposing power.
So that encourages players to keep some units on the border….because they will defend as if they were tanks on the first round.
Or an AAA would defend at ‘2’ on the first round.
Think trench warfare…both those powers were expecting an attack on that front (defensive posture) but didn’t have the initiative, or reason, to launch an attack.