The Three Hundred and Thirty-Five Year War was one of the longest in history.
It was fought between the Netherlands and a small group of English islands called the Isles of Scilly. The war occurred from 1651 – 1986. No shots were fired and the two territories forgot they were ever even at war.
In fact, it is disputed whether there ever really was a war in the first place. Nevertheless the two territories signed a peace treaty in 1986, just in case.
The Dutch Ambassador at the time joked that it must have been horrible for the Scillians knowing that they could have been attacked at any moment.