Two nuclear weapons were used by the US against Japanese cities at the end of World War 2 :
On August 6, 1945, at 8:15 AM Tokyo time, a B-29 bomber, the "Enola Gay", piloted by Paul W. Tibbets, dropped a uranium atomic bomb, code named "Little Boy" on Hiroshima, Japan's seventh largest city. In the blast, thousands died instantly. The city was unbelievably devastated. In the aftermath, a firestorm of winds followed the blast as air was drawn back to the center of the burning area. Winds of over 40 mph uprooted large trees around the city.
That same day, August 6, the only other A-bomb on the planet was assembled at Tinian Island. Confusion among the Japanese leadership meant that despite the destruction of Hiroshima, no surrender agreement was reached. On August 8, Field Order No.17 issued from the 20th Air Force Headquarters on Guam called for its use the following day on either Kokura, the primary target, or Nagasaki, the secondary target.
On August 9, another B-29 bomber, "Bockscar", piloted by Col. Charles Sweeney, reached the sky over Kokura, but the city was obscured from view. The plane turned toward Nagasaki.
Nagasaki was an industrialized city with a natural harbor in Western Kyushu, Japan. At 11:02 a.m., this bomb, a plutonium bomb known as "Fat Man" , exploded over the north factory district at 1,800 feet above the city, an airburst like Hiroshima to achieve maximum blast effect. The hills around Nagasaki spared most of the city from the worst effects of the blast, which still did incredible damage. Flash burns from primary heat waves caused most of the casualties to inhabitants. Others were burned when their homes burst into flame. Flying debris caused many injuries. The bomb took the lives of 42,000 persons and injured 40,000 more. It destroyed 39 percent of all the buildings standing in Nagasaki. According to U.S. estimates, 40,000 people were killed or missing as a result of the second bomb.