William, the duke of Normandy: 1048 to 1060

This is not a bad piece of military strategy, because Ponthieu is leaderless now, with count Guy in your dungeon: but it isn't good political strategy. You can hold onto the county only by committing a large number of troops.

Then shortly after you take over Ponthieu, king Henry and count Geoffrey invade Normandy with the biggest army you have ever seen. Defending the duchy will require all your resources. It will be a mad scramble for you to get out of Ponthieu in time to meet this enormous threat: time to flip that coin:
Heads you win and get back in time.
Tails you lose the war.