In 1942, The first atomic pile, a sustained controllable nuclear chain reaction, came online in Chicago. Scientist and inventor Enrico Fermi remarked, "This will be remembered as the darkest day in history," referring to both the atomic pile and that day's announcement of Nazi death camps operating in occupied Europe. Actually, most people have no knowledge of that day; they remember the ultimate achievement that began on that date - the atomic bomb.
The Manhattan Project - named because the first team started working in Manhattan - was started before the war began at the urging of Einstein and other scientists. The warning was terrifyingly simple - the best theoretical physics was being done in Germany. A weapon of unimaginable power is possible. If the Nazis get it first, it won't matter what the size is of the Allied Armed Forces, they could be annihilated in nuclear fire.
But raids on the German uranium and heavy water production facilities showed they were far behind the American efforts. A comprehensive facility, secretly built in Los Alamos, was administrated by US Army General Leslie Groves and managed by civilian scientist Robert Oppenheimer. By 1944, they were developing an atomic weapon that would deliver a knockout blow to Nazi Germany.
But as the spring of 1945 ended, and the bomb moved from theory to reality, the scientists began to question whether it was necessary to develop a weapon. When the bomb was ready for testing in July 1945, a group of scientists, led by Leo Silzard, questioned if dropping the bomb was needed at all.
But the decision was already being made in favor of dropping the weapon on Japan. It was prepared for the B-29, a plane only operating in the Pacific Theatre of Operations. In Utah, The 504th composite group began practicing on dropping one bomb from a high altitude and turning around quickly. The pilots were sworn to secrecy without even knowing what the secret was.
On July 14, 1945, the bomb was detonated in a test in New Mexico. The scientists had no idea of how much explosive power the bomb would have; ideas ran from a dud to setting the atmosphere on fire. Edward Teller, after the war the father of American nuclear doctrine, had the highest guess: 1,000,000 tons of TNT. The bomb had the power of 20,000 tons of TNT. A cover story was that a munitions depot exploded. A blind woman claimed to see the atomic light from miles away; unfortunately, that was the last time anyone connected with nuclear weapons saw the light through blind eyes.