"Need for War" is the name for the decision process AI wizards use to Break Treaty or Declare War towards others, including the human player. To see when this happens, refer to AI to Player Diplomacy or AI to AI Diplomacy. In order for anything to happen at all, the target of the check must not be the human player, or the game has to progress into turn 100 or later. Additionally, neither wizard can be banished, they cannot already be at War with each other, and they must have Diplomatic Contact.