My candidate is the Shortest War Ever Fought, detailed under :
Youtube film on the "shortest war ever" :
The shortest war ever fought in the United States lasted only one day and took place against the self proclaimed "Free State Of Van Zandt" right here in East Texas. While there are many versions of the story, Alvis Allen, Van Zandt County Geneological Society Historian, has researched the facts and the legends.
5 Interesting Facts About England / THE SHORTEST WAR EVER!!!
On December 11th 1845 the Khalsa crossed the river, and Hardinge declared war. So began what George Macdonald Fraser’s Flashman called “the bloodiest, shortest war ever fought in India”. It was to be a war of battles.