The appropriately named 335 Year War was a conflict between the the Netherlands and the Isles of Scilly that did not cause any deaths. The two nations declared war on each other in 1651 and, quite simply, forgot to declare peace until 1986.
The 335 Year War came as the result of British Civil War, which pitted two groups against each other: the Parliamentarians and the Royalists. The Netherlands wanted to be on the winning side, and the Parliamentarians were driving Royalists out of Britain and to the Isles of Scilly to the south.
Naturally, the Netherlands bet on the Parliamentarians and joined their side. It proved to be a smart move, even though the Netherlands and the Royalists had been allies for a very long time up to this point. Betrayed and angry, the Royalists raided and looted Dutch trade ships in the English Channel. In response to the attacks, the Dutch sent war ships to the Isles of Scilly to demand reparations for the raids. When the Royalists refused, the Dutch declared war. Thus began the longest and least painful war in human history.
Three months later, the Royalists surrendered to the Parliamentarians. The British Civil War had ended, and the Netherlands navy returned home from Scilly without a scratch. They forgot one important technicality: they never signed a formal peace treaty with the Royalists and the Isles of Scilly.
1985, a historian from Scilly, Roy Duncan, noticed this oddity and wrote to the Dutch Embassy in England. Both sides were shocked to learn that they were "still at war." And so, in 1986, seeking to resolve the sordid bloodless affair, Scilly and the Netherlands signed a peace treaty.
Dutch Ambassador Jonkheer Rein Huydecoper, upon signing the agreement in Scilly, remarked that "it must have been harrowing to the Scillonians to know we could have attacked at any moment." With that, the longest "war" in human history had ended.