War refers to a state of officially declared war under alliance options. War can only be declared by those who have been given permission to declare war by the alliance founder/leader. Though war can be declared by only one party the other party who has had war declared upon has the option to accept the declaration of war.
Often, war will only be declared if one alliance wants to destroy another. Members of different alliances may raid each other without war being declared, as they want resources and don't fear the other alliance. This can, however, escalate. An attack with catapults or chiefs is very often seen as a declaration of war, as the motive behind it is either desire to capture a village or to destroy it. Therefore, alliances usually don't allow this unless they are ready for a war or the target is outside an alliance.