The Mouse That Roared (1959)

The Duchy of Grand Fenwick decides that the only way to get out of their economic woes is to declare war on the United States, lose and accept foreign aid. They send an invasion force to New York (armed with longbows) which arrives during a nuclear drill that has cleared the streets. Wandering about to find someone to surrender to, they discover a scientist with a special ultimate weapon that can destroy the Earth. When they capture him and his bomb they are faced with a new possibility: What do you do when you win a war?