The 27th Day (1957)

Five individuals from five nations, including the USA, USSR, and China, suddenly find themselves on an alien saucer, where an alien gives each a container holding three capsules. The alien explains that no power on earth can open a given container except a mental command from the person to whom it is given, then anyone may take a capsule and, by speaking a latitude and longitude at it, cause instant death to all within a given radius: thus, each of the five has been provided with the power of life and death. Then, they are given 27 days to decide whether to use the capsules, and returned to the places from which each one came.