Through diplomacy, realms can declare war on each other.
War can be unilateral or mutual. A unilateral war is when one realm declared war on another but the other realm does not declare war back.
War can be declared starting on day 5 of the round.
In a mutual war, the bonuses are +10% OP and +20% land gains and these bonuses become active the moment mutual war is declared, irrespective of how recently the initial war declaration occurred.
By declaring war, dominions in both realms can perform black-ops. This takes place immediately upon declaring war and is not delayed by 24 hours, unlike the OP and land gains bonuses.
In a mutual war, successful black-ops also cause a prestige gain for the dominion performing the ops and a prestige loss for the target.
Black ops are available to be used starting on day 8 of the round.