Between 1937 and 1941 Japan conquered and occupied most of East and Southeast Asia.  In December 1941 Japan attacked the US Navy in Pearl Harbor, prompting US entry into the war and widening the scope of the conflict.  However, the Japanese had limited resources for a lengthy war, especially as the US and Britain began pushing the Japanese out of East and Southeast Asia.

US scientists successfully tested the first atomic bomb in the New Mexico desert on July 16, 1945, while President Truman was meeting with Stalin and Churchill at Potsdam.  The allies issued the Potsdam Declaration calling upon Japan to surrender or suffer “the utter devastation of the Japanese homeland” and concluding with a call for the Japanese government “to proclaim now the unconditional surrender of all Japanese armed forces” (T. E. Vadney, The World Since 1945, p. 44).  This reiterated the call for Japan’s unconditional surrender first issued in 1943.  By the time of the Potsdam Declaration, Truman had already ordered the US Air Force to use the bomb as soon as possible after August 3.  The first bomb was dropped on Hiroshima August 6; the Soviets declared war on Japan August 8; and a second bomb was dropped on Nagasaki August 9.  The Japanese notified the US of their intention to surrender on August 10, initial papers were signed August 15, and the formal surrender ceremony took place September 2.  Even after both bombs were dropped, “unconditional surrender” was not obtained because the Japanese Emperor was allowed to maintain his throne, the main issue of contention for the Japanese.

Howard Zinn criticizes Truman’s decision to drop the bombs, arguing that the main reason he did so was to keep the Russians out of Japan.  At the Yalta Conference the Soviet Union agreed to invade Japan no later than three months after the end of the war in Europe.  The war in Europe ended on May 9, 1945, and the US dropped the first bomb on August 6, 1945, according to Zinn, because they wanted to end it before the Soviets entered.  Otherwise, the US might have to share the postwar occupation of Japan with the Soviet Union, as in Germany and Korea.  Zinn, A People’s History of the United States.

SYSTEMATIC DESTRUCTION
The firebombing campaign, which began in earnest with the great raid against Tokyo on the night of March 9-10, proved far more devastating than expected. During the next five months, [US Air Force Secretary] LeMay's bombers razed one half of the total area of 66 cities—burning 178 square miles.  By the summer of 1945, Japan's productive capacity had been lowered as follows:  power generation by 50 percent, oil by 85 percent, and overall industrial production by 60 percent. The destruction was so complete LeMay warned his superiors he would run out of targets by September.

After the war, Truman often said that the invasion of Japan could have cost half a million or a million American casualties.  The origin of these figures is uncertain, but Truman knew that Japan had some two million troops defending the home islands.  He believed, along with the many Americans who would have had to invade Japan, that such a campaign might have become, in his words from June 18, 1945, "an Okinawa from one end of Japan to the other."  Added to the American losses would have been several times as many Japanese casualties—military and civilian.  The Allies and Asian countries occupied by Japan would also have lost many additional lives.  For Truman, even the lowest of the casualty estimates was unacceptable.  To prevent an invasion and to save as many lives as possible, he chose to use the atomic bomb.

However, other postwar observers, including Secretary Stimson, doubted that Japan's rulers would have accepted unconditional surrender if the home islands had not been invaded or if the atomic bomb had not been dropped.  In any case, many American lives would have been lost by November 1, 1945, and after that date, the invasion of Kyushu would have been in full swing.