phoenix
About an hour before the bombing, the Japanese early warning radar net detected the approach of some American aircraft headed for the southern part of Japan. The alert had been given and radio broadcasting stopped in many cities, among them Hiroshima. The planes approached the coast at a very high altitude. At nearly 08:00, the radar operator in Hiroshima determined that the number of planes coming in was very small?probably not more than three?and the air raid alert was lifted (to conserve fuel and aircraft, the Japanese had decided not to intercept small formations).
http://en.wikipedia.org/wiki/Atomic_...a_and_Nagasaki