On August 6, 1945 the United States Army Air Forces dropped the atomic weapon "Little Boy" on the Japanese city of Hiroshima, becoming the first and only nation to have used nuclear weapons in war to date. The bomb released a mix of shock waves, heat rays and radiation, causing widespread devastation. The city was instantly flattened and an estimated 140,000 of the total 350,000 people who lived there were killed or died within months after the American B-29 bomber Enola Gay dropped the bomb.

Three days after the attack on Hiroshima, the United States dropped a second atomic bomb, "Fat Man," on the southern Japanese city of Nagasaki, killing some 80,000 people. Japan surrendered to the Allies six days later, marking the end of World War II.

The invention of the atomic bomb and the drive for the Unites States to build it during the war against Nazi Germany, lay largely with a group of Jewish scientists. Scientific genius Albert Einstein's theory of relativity, and particularly his famous equation E=mc² explaining the relationship between mass and energy, was at the root of the development of the nuclear bomb. But it was physicist Leo Szillard who conceived the nuclear chain reaction in 1933, and together with Italian Catholic physicist Enrico Fermi, patented the idea of a nuclear reactor.

An infamous letter delivered to then US president Franklin Roosevelt, signed by Albert Einstein on August 2, 1939, and written at the request of Szillard and physicist Eugene Wigner, is widely cited as the spark that ignited the flame - the Manhattan Project. These two scientists, with the help of Edward Teller, visited Einstein to persuade him – as the generation's most renowned scientist in the world - to sign the letter urging Roosevelt to support the construction of an atomic bomb. Their motive to create the atomic weapon was born out of fear that Nazi Germany would develop the weapon first, the same fear that persuaded former pacifist Einstein, to pen the letter.
 
"In the course of the last four months it has been made probable -through the work of [Frédéric] Joliot in France as well as Fermi and Szilard in America - that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future. This new phenomenon would also lead to the construction of bombs, and it is conceivable - though much less certain - that extremely powerful bombs of a new type may thus be constructed," the letter read.

Addressing concerns regarding Germany, the letter continued, "I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institute in Berlin where some of the American work on uranium is now being repeated." American economist Alexander Sachs delivered the letter to Roosevelt, and according to some accounts, verbally summarized the letter for the president out of concern that he would not look at it otherwise.  

On June 28, 1941, Roosevelt signed Executive Order 8807, which created the Office of Scientific Research and Development, in order to coordinate scientific research for military purposes. One unit of this was the top-secret "S-1 Section," later to become known as the Manhattan Project.

American physicist J. Robert Oppenheimer was appointed director of the Manhattan Project at Los Alamos in 1942, and was subsequently dubbed the "father of the atomic bomb." He selected the scientific team, which included Fermi, Hans Bethe, Edward Teller, Victor Weisskopf and Richard Feynman among others. Canada and the United Kingdom also participated in the US-led project.

Einstein played a minor role in the creation of the nuclear weapon, sending several subsequent letters to Roosevelt, and helping with a specific problem involving the separation of isotopes that shared chemical traits. Due to his pacifist past, Einstein was considered to be too much of a security risk to be included in the project. And on August 6, 1945, Einstein was devastated upon hearing the news that the US had dropped an atomic bomb on Hiroshima.

The sought-out scientist had agreed to the correspondence with Roosevelt under the conviction that a nuclear weapon would serve as an effective deterrent against Nazi Germany. He lived to regret his part in the creation of the deadly weapon: "I made one great mistake in my life when I signed the letter to President Roosevelt recommending that atom bombs be made," Einstein confessed in 1954, just five months before his death. "But there was some justification - the danger that the Germans would make them," he countered.

Then-US president Harry S. Truman, however, had no regrets. He maintained that the move was necessary to end the war as quickly as possible with as few casualties as possible. Following a rare, positive column in the Chicago Sun Times about Truman's use of the atomic bomb, he replied to columnist Irv Kupcinet in a letter: "It (the bombing) was done to save 125,000 youngsters on the American side and 125,000 on the Japanese side from getting killed and that is what it did… I have no regrets and, under the same circumstances, I would do it again."

Historian, activist and writer Gar Alperovitz has notoriously challenged this justification, arguing that the use of the deadly weapons were unnecessary, that there was "very substantial evidence that by that late summer of 1945 the decision was primarily influenced by diplomatic considerations related to the Soviet Union" following the end of the war.

Three months after "Little Boy" and "Fat Man" were dropped on Hiroshima and Nagasaki, Oppenheimer resigned as project director, and opposed the development of the even more deadly thermonuclear bomb. As he, Einstein and other scientists advocated nuclear disarmament, Teller adopted the role of the father of the Hydrogen Bomb along with Stanisław Ulam, and successfully developed the weapon in 1951.

While several countries in the world have since developed nuclear weapons, the Truman Administration remains to date, the only government in history to have given the green light to drop the deadly bombs. The issue of nuclear weapons is today more pertinent than ever, as Western powers and particularly Israel, continue an ongoing battle to prevent the Iranian regime from joining the list of nuclear-armed states.

"What we have today is far more powerful," the last surviving member of the crew that dropped "Little Boy" warned in an interview with the BBC in 2010. Flight navigator Lieutenant Theodore "Dutch" Van Kirk said that the bombs they used, "were midgets compared to those that exist today. Nuclear bombs…can destroy the world if they are released. I would hope that atomic bombs will never again be used, for any reason."

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