For its part, the Soviet Union harbored fears of invasion. Having suffered at least 27 million casualties during World War II after being invaded by Nazi Germany in 1941, the Soviet Union was wary of its former ally, the United States, which until late 1949 was the sole possessor of atomic weapons. The United States had used these weapons operationally during World War II, and it could use them again against the Soviet Union, laying waste its cities and military centers. Since the Americans had a much larger air force than the Soviet Union, and the United States maintained advance air bases near Soviet territory, in 1947 Stalin ordered the development of intercontinental ballistic missiles (ICBMs) in order to counter the perceived American threat.