Officially, the longest war in history was between the Netherlands and the Isles of Scilly, which lasted from 1651 to 1986. And history says there were no casualties.
The war is said to have been extended by the lack of a peace treaty for 335 years without a single shot being fired, which would make it one of the world’s longest wars and a bloodless war. Despite the uncertain validity of the declaration of war, and thus uncertainty about whether or not a state of war ever actually existed in the first place, peace was finally declared in 1986.
Sources: wikipedia, list25.