What was the longest war in history?

The answer depends on whether you count declared wars or actual fighting. The most cited technical record belongs to the Three Hundred and Thirty Five Years' War between the Netherlands and the Isles of Scilly, a small archipelago off Cornwall. A Dutch admiral declared war on the Royalist-held islands in 1651 during the English Civil War, the conflict was forgotten when the Royalists surrendered months later, and no peace was ever signed. A local historian rediscovered the omission in the 1980s, and a Dutch ambassador signed a treaty ending it in 1986. Not a single shot was fired in all that time.

For sustained actual conflict, the Reconquista, the series of campaigns by Christian kingdoms to take the Iberian Peninsula from Muslim rule, ran from the early 700s to the fall of Granada in 1492, roughly 780 years, though it was a sequence of wars and truces rather than a single continuous one. The Roman-Persian wars stretched across about 680 years with long intervals of peace. The Hundred Years' War between England and France, the best known long war, actually ran 116 years from 1337 to 1453 and was likewise a chain of separate conflicts.

Two more modern cases are worth knowing. Korea remains technically at war, since the 1953 armistice suspended fighting without a peace treaty, making the conflict over 70 years old on paper. And the Anglo-Zanzibar War of 1896 holds the opposite record as the shortest, lasting somewhere between 38 and 45 minutes before Zanzibar's palace surrendered to British bombardment.