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.
In a mutual war, the bonuses are +10% OP and +20% land gains.
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.