"We have discovered the most terrible bomb in the history of the world." President Truman
"The damn thing will never work." William Leahy, White House Chief of Staff.
Truman was right. The atom bombs that exploded above Hiroshima and Nagasaki in August 1945 were devastatingly effective. Thousands of buildings disintegrated. Flying birds ignited as ground zero temperatures reached 5000 degrees. An estimated 375,000 people died—mostly civilians, although Truman had insisted that the bombs not be used on women and children.
Since then, the world has debated not only the targets, but whether the bombs had to be dropped at all. There was little debate at the time. The Allies spent two billion dollars (about $26 billion today) developing The Bomb, not sure it would work. An awesomely successful test near Alamogordo, New Mexico, proved otherwise. Some still argued that Japan was collapsing, and with the planned invasion of the Japanese mainland still three months away, the Allies could afford to wait, especially as massive incendiary bombing of Japanese cities continued. (During ten days in March, 11,600 B29 sorties killed 150,000 Japanese.)
A-bombing proponents claimed that some Japanese leaders were eager for national suicide. ("Would it not be wondrous for this whole nation to be destroyed like a beautiful flower?" General Anami.) There was evidence of that sentiment in kamikaze attacks, battles to the death on Iwo Jima and Okinawa, and Japanese recruits being trained as human bombs. A million Yanks were preparing for a mainland invasion, and estimates were that 250,000 men could be casualties in just the first month.
Perhaps more than any other reason for dropping the bombs, America was tired of the war and wanted it over quickly. Victorious European GIs didn't want to be reassigned to the Pacific, and their families concurred. Truman allowed the attacks because delaying the war solely to save the Japanese a "most terrible bomb" would have been political suicide. So Hiroshima (because it had military bases, shipyards, industrial plants, and no Allied POW camps) and Nagasaki (because prime target Kokura was fogged in) were destroyed. Emperor Hirohito immediately began negotiating surrender terms. Truman thought a million lives were saved, while the Manhattan Project's General Groves told Congress, "Radiation poisoning is a very pleasant way to die."
Decades later, Japanese were still dying from radiation.