During the English Civil War (1641 – 1651), the Parliamentarians won a string of victories against the Royalists, pushing them farther and farther away from London. Eventually, the Royalist fleet was forced to retreat to the remote Isles of Scilly, an archipelago southwest of Cornwall.
Despite the fact that they had been allied with the British crown, the Dutch sided with the Parliamentarians. The Royalists felt betrayed and made a concentrated effort to raid Dutch shipping. This caused considerable losses, and the Dutch decided to retaliate and demand compensation.
However, they couldn’t demand it from England since they had been attacked by the Scilly Royalists rather than the Parliamentarians, who had by then taken control over most of England. Therefore, the Netherlands demanded compensation from the Isles of Scilly, who refused to pay. The Dutch declared war on the Isles on 30 March 1651 and set up a naval blockade.
Later that year, the Parliamentarians drove the Royalists from the Isles of Scilly, so the Dutch had no reason to continue hostilities. The war was forgotten until 1985, when Roy Duncan, a local historian and the chairman of the council that governs the Isles, discovered that no peace treaty had been signed between the Isles and the Dutch.
It was soon agreed that a peace treaty would be signed, and this happened on 17 April 1986, 335 years after hostilities were declared. This makes it one of the longest wars in history. Interestingly, the war was also completely bloodless.
It should be noted, however, that there are some doubts whether the war was legitimate. For one, war was declared by Dutch admiral Maarten Tromp, and it remains unclear whether he actually had the authority to do so. Another cause for doubting the authenticity of the conflict is that it is technically impossible to declare war on a specific region of a sovereign state.