The 27th Day (1957) Gene Barry, Valerie French, George Voskovec

An alien from a dying planet brings five average people from England, Germany, China, the Soviet Union and the United States onto its spacecraft. Each person is given a case containing vials that can destroy all life within a 3000-mile radius. They’re told that the Earth will be spared complete destruction if all five can keep their respective governments from using the capsules for 27 days, but escalating Cold War tensions between the Americans and the Russians make that a difficult promise.