What was the longest war?
Between declaring war and signing a peace treaty?
If you are speaking of the US the longest conflict has been in Afghanistan, the longest "declared" war was the War for independence (7 years).
I heard The Hundred Years War was pretty long.
it was the threehundred and thirtyfive year war the Netherlands fought some island
The Three Hundred and Thirty Five Years' War ~http://en.wikipedia.org/wiki/Three_Hundred_and_Thirty_Five_Years%27_War