Before the U.S. entered WWII, they had already begun conducting research into the development of atomic weapons. In 1940 the U.S. government began funding an atomic weapons development program in response to nuclear weapons research being conducted in Nazi Germany. The U.S. Army Corps of Engineers constructed vast facilities for the top-secret program, which was known as the Manhattan Project. Construction of the atomic bomb took place in Los Alamos, New Mexico with the team being led by J. Robert Oppenheimer.
On July 16, 1945, the Manhattan Project held its first successful test of a plutonium bomb at the
Trinity test site at Alamogordo, New Mexico.
By the time of the Trinity test Allied powers had defeated Germany in Europe, however the fighting was not over. Japan was refusing to surrender and in 1945 had inflicted half the casualties on the Allies that they had suffered in three full years of the war in the Pacific. The Japanese never intended to defeat American forces, just to inflict massive casualties and break the resolve of the American public. They also began preparing themselves for an American invasion. On July 26, 1945, United States President Harry S. Truman, United Kingdom Prime Minister Winston Churchill, and President of China Chiang Kai-shek issued a document known as the Potsdam Declaration, which demanded Japan’s surrender and threatened “prompt and utter destruction” if they refused.
Japan refused to surrender and General Douglas MacArthur and other top military commanders suggested response was “Operation Downfall”. This involved continuing the conventional bombing of Japan and following it up with a massive invasion. The commanders advised Truman that it would result in up to 1 million casualties. Truman wanted to avoid such a high casualty rate and despite the moral reservations of Secretary of War Henry Stimson, General Dwight D. Eisenhower, and others on the Manhattan Project, he decided to use the atomic bomb in the hopes of bringing the war to a quick end. He believed that the atomic bomb would end the war and give the U.S. a dominant position in the postwar world.
Truman wanted to avoid extreme civilian casualties and so the first location chosen for the dropping of an atomic bomb was a military base/manufacturing center, Hiroshima. Hiroshima had a population of ~350,000 and was ~500 miles from Tokyo. The Uranium-235 bomb, “Little Boy”, was dropped at 8:15 am on August 6 and exploded 2,000 feet above Hiroshima. The blast was equal to 12-15,000 tons of TNT and destroyed 5 square miles of the city. The temperature at ground level reached 7,000 degrees Fahrenheit in less than a second. At least 80,000 people died instantly, some being vaporized as far away as half a mile from ground zero. Bronze statues melted, roof tiles fused together, and people miles away had their skin burned from the infrared energy released. There were fires, smoke, and bubbling hot tar everywhere and it is estimated that as many as 135,000 people died in Hiroshima, many dying later from injuries and radiation poisoning.
Despite this destruction, Japan still refused to surrender. Historians claim that even if the Emperor had said they should surrender after Hiroshima, he could not have been 100% certain that the armed force would comply. The Imperial Army responded to the bombing by saying that they will not concede that the Americans have an atomic bomb until an investigation is completed. The Imperial Navy claimed that even if the Americans had an atomic bomb, it could not be that powerful and they could not have that many. Ultimately the armed forces would only be impressed if the U.S. had an arsenal of atomic weapons.
With no surrender, the U.S. decided to drop another bomb. Their initial target, the city of Kokura, was covered by clouds, and so the bomb was dropped on the secondary target, Nagasaki. The plutonium bomb “Fat Man” was dropped at 11:02 am on August 9, and was more powerful than “Little Boy” with the ability to produce a 22-kiloton blast.
The destruction was limited to 2.6 square miles because of the narrow valleys and mountains surrounding Nagasaki. 40,000 people were killed instantly and another 60,000 were injured, with tens of thousands more dying of long-term effects of the radiation blast. On August 15, 1945, Emperor Hirohito announced the country’s surrender in a radio broadcast. Initially the overseas commanders refused to surrender, and it took extra efforts to convince them to eventually comply. The formal surrender agreement was signed on September 2, on the U.S. battleship Missouri, which was anchored in Tokyo Bay.
To this day there is still debate on the use of atomic weaponry and its effectiveness as a deterrent. Truman claimed that he was faced with a series of terrible choices and he chose the least abhorrent. He would remain steadfast that his decision was the right decision, though he felt a deep responsibility over the consequences of the choice. Many of the scientists involved in the Manhattan project struggled with the role they played in the development of the bomb. Oppenheimer himself was concerned that the decision to use the weapons in Japan was not correct. He believed that Japan did not really understand the destruction the weapons could cause when the Potsdam Declaration was issued. He felt he had blood on his hands, and his relationship with Truman deteriorated.