It puts me in mind of "The Mouse That Roared".
The minuscule European Duchy of Grand Fenwick is bankrupted when an American company comes up with a cheaper imitation of Fenwick's sole export, its fabled Pinot Grand Fenwick wine. Crafty prime minister Count Mountjoy (Peter Sellers) devises a plan: Grand Fenwick will declare war on the United States, then surrender, taking advantage of American largesse toward its defeated enemies to rebuild the defeated nation's economy. Grand Duchess Gloriana (also Sellers) is hesitant but agrees to the plan. Mild-mannered game warden Tully Bascomb (also Sellers) is charged as Field Marshal to lead the Fenwick troops, aided by Sergeant Will Buckley (William Hartnell).
The contingent of 20 soldiers in medieval chain mail books passage across the Atlantic on a small merchant ship, arriving in New York Harbor during an air-raid drill that leaves the city deserted and undefended. They chance upon a civil defence truck and are mistaken for invading Martians, prompting an investigation by blustering but ineffectual General Snippet (MacDonald Parke). Puncturing the tyres of the general's jeep with their bows and arrows, the Fenwick troops take him and four NYPD officers hostage. Still looking for a place to surrender, Tully and Will stumble across Dr. Alfred Kokintz (David Kossoff), whose invention of the Q Bomb – capable of destroying an entire continent – has prompted the defense drills. He has built a football-sized prototype of the unstable bomb, which Tully takes possession of. With Kokintz and his attractive daughter Helen (Jean Seberg) as additional hostages, Tully declares victory and returns with them to Grand Fenwick.
The duchess indulges Tully's victory, and the prime minister resigns in disgust at Tully's blunder, leaving him as acting prime minister. When the incident is discovered, the U.S. government is thwarted from retaking the weapon and hostages by force, fearing the dishonor of attacking such a small and defenseless nation. Instead, they send the U.S. Secretary of Defense (Austin Willis) to discuss terms of surrender and get back the bomb. Meanwhile, Grand Fenwick receives competing offers of defensive aid from each of the world's powers, in exchange for the weapon.
Tully becomes smitten with Helen, who initially despises him for taking them but falls for his simple charm. Then Snippet and Mountjoy conspire to steal the bomb and return it (and Helen) to America, but Tully gives chase and retrieves it. The Secretary of Defense and Tully agree to terms: the knock-off wine will be taken off the market, Grand Fenwick will receive monetary aid from the US, Helen and her father will remain in Grand Fenwick, and so will the bomb: held by "the little countries of the world" as a weapon of last resort if the superpowers refuse to disarm.
Checking the bomb for damage, Tully, Helen, and Dr. Kokintz find it was "a dud" all along, and leave it in the dungeon, conspiring to keep its impotence secret. However, after they leave, a mouse emerges from it, and it appears to rearm, sitting ready to explode if disturbed.