It would completely depend on how hostile the surrounding factions were. In the case of Valhalla and Englad I would like conquer in response to the open hostility.
Those willing to cooperate and work together would receive a diplomatic approach.
Diplomacy in an Assassin's Creed game? HA! The only way is the conquering way. Look, we always get backstabbed in the story, why would anyone try to be diplomatic?