After the German surrender in May, Japan was left with the Allies' undivided attention. The atomic bombings of Hiroshima and Nagasaki, which took place on August 6 and August 9, 1945, forced a Japanese surrender without costing the Allies a bloody invasion of the home islands. The two bombings, which remain the only nuclear attacks in history, ushered in a new age and permanently changed the nature of warfare, politics, and international diplomacy.
The roots of the atomic attacks stretched back to 1939, when a group of scientists, including the famed physicist Albert Einstein (1879-1955), concerned that Germany was developing a program to build atomic weapons, approached President Franklin Roosevelt. Such weapons, the scientists argued, were a theoretical possibility, and the United States would do well to form an exploratory committee with an eye towards developing such technology before the Nazis did.
The Manhattan Project
Roosevelt took their advice, and the exploratory committee developed into the Manhattan Project, a top-secret government effort that funneled $2 billion into building an atomic weapon. Under the guidance of a brain trust of top physicists, the theoretical possibility was on the verge of becoming an atomic reality when President Roosevelt died in April 1945.
Newly sworn-in President Harry Truman was then told of the bomb and its destructive potential. He was further informed that the bomb would be ready in four months. As it turned out, Germany did not last that long, surrendering on May 8. With the intended target of the bomb now out of the war, the question of whether to use the weapon on Japan quickly arose.
The Potsdam Declaration
At the victorious Allies' German Potsdam Conference in July 1945, Truman warned Japan to surrender immediately or face "utter devastation," although he did not provide any further details. Meanwhile, in Alamogordo, New Mexico, the first atomic bomb was successfully tested on July 16.
August 1945 was the decisive month of the war in the Pacific. The Soviet Union, in accordance with agreements reached at Yalta, was gearing up to enter the war against Japan. Whether this factored into the American decision to use the atomic bomb is unknown.
What is certain is that the main reason for using the bomb was the goal of a quick surrender of Japan, which would eliminate the need for an invasion of the home islands. Such an invasion was set to begin in November. After the bloodbath on Okinawa, the prospect of fighting two million determined Japanese defending their homeland, backed by 5,000 or more kamikaze fighters, motivated the Americans to avoid an action that would likely result in millions of deaths and total Japanese casualties in the tens of millions.
Several options for demonstrating the bomb's capabilities in a non-lethal way—detonating it in front of a panel of international observers or dropping it into Tokyo Bay—were dismissed because a detonation failure would only strengthen Japanese resolve.
Truman authorized the use of the bomb in early August. A target committee had selected several cities that were both military and psychological targets, and from this list the city of Hiroshima, an important military-industrial center, emerged as the primary target, in part because it was the only city on the list without a POW camp.
Hiroshima and Nagasaki
In the early morning hours of August 6, 1945, the B-29 Enola Gay took off from its base on Tinian with an escort of two other bombers carrying instrumentation and photography equipment. By 08:15 a.m., the bombers were over Hiroshima and the bomb was released. The blast, equivalent to 12,500 tons of TNT, created a fireball that reached 5,400 degrees Fahrenheit and killed around 70,000 people instantly. Outside the one-mile blast radius, fires quickly began to spread, eventually burning down four square miles of the city.
On August 8, as authorities in Tokyo began to slowly appreciate what had just happened, the Soviet Union entered the war, invading Manchuria and scything through the Japanese Kwangtung Army stationed there. The following day, ahead of a predicted weeklong period of bad weather, a second bomb was hurriedly dropped on the city of Nagasaki. The second blast was somewhat contained by the hills around the epicenter; at least 40,000 people were killed outright, including some survivors of the Hiroshima blast who had fled that city three days before.
The Japanese government, which had been making conditional peace overtures through Moscow, agreed to a near-unconditional surrender at the behest of Emperor Hirohito. The only condition the Japanese now insisted on was the preservation of the Imperial line. This was agreed to and Hirohito made a radio address on August 14—after a militarist coup attempted to stop the broadcast—announcing Japan's capitulation and asking his disbelieving subjects to "endure the unendurable."
The relative roles that the atomic bombs and the Soviet invasion played in the Japanese decision to surrender have been a source of endless debate. Even Japanese officials, in postwar interviews, seemed to give conflicting assessments. Regardless of the effectiveness of the atomic attacks, the suffering they unleashed cannot be denied.
In the confused hours after the attack on Hiroshima, Radio Tokyo supplied some of the first accounts of the aftermath of the bombing:
With the gradual restoration of order following the disastrous ruin that struck the city of Hiroshima in the wake of the enemy's new-type bomb on Monday morning, the authorities are still unable to obtain a definite checkup on the extent of the casualties sustained by the civilian population. Medical relief agencies that were rushed from neighboring districts were unable to distinguish, much less identify, the dead from the injured. The impact of the bomb was so terrific that practically all living things, human and animal, were literally seared to death by the tremendous heat and pressure engendered by the blast. All the dead and injured were burned beyond recognition. With houses and buildings crushed, including the emergency medical facilities, the authorities are having their hands full in giving every available relief under the circumstances. The effect of the bomb was widespread. Those outdoors burned to death while those indoors were killed by the indescribable pressure and heat.