According to John Dower, in his book Cultures of War: Pearl Harbor/Hiroshima/9-11/Iraq, the factors behind the success of the occupation were:
Discipline, moral legitimacy, well-defined and well-articulated objectives, a clear chain of command, tolerance and flexibility in policy formulation and implementation, confidence in the ability of the state to act constructively, the ability to operate abroad free of partisan politics back home, and the existence of a stable, resilient, sophisticated civil society on the receiving end of occupation policies – these political and civic virtues helped make it possible to move decisively during the brief window of a few years when defeated Japan itself was in flux and most receptive to radical change.
Japan surrendered to the Allies on August 14, 1945, when the Japanese government notified the Allies that it had accepted the Potsdam Declaration. On the following day, Emperor Hirohito announced Japan's unconditional surrender on the radio (the Gyokuon-hōsō). The announcement was the emperor's first ever planned radio broadcast and the first time most citizens of Japan ever heard their sovereign's voice. This date is known as Victory over Japan, or V-J Day, and marked the end of World War II and the beginning of a long road to recovery for a shattered Japan. Japanese officials left for Manila, Philippines on August 19 to meet MacArthur and to be briefed on his plans for the occupation. On August 28, 1945, 150 US personnel flew to Atsugi, Kanagawa Prefecture. They were followed by USS Missouri, whose accompanying vessels landed the 4th Marine Division on the southern coast of Kanagawa. Other Allied personnel followed.
MacArthur arrived in Tokyo on August 30, and immediately decreed several laws. No Allied personnel were to assault Japanese people. No Allied personnel were to eat the scarce Japanese food. Flying the Hinomaru or "Rising Sun" flag was initially severely restricted (although individuals and prefectural offices could apply for permission to fly it). This restriction was partially lifted in 1948 and completely lifted the following year.
On September 2, 1945, Japan formally surrendered with the signing of the Japanese Instrument of Surrender. On September 6, US President Truman approved a document titled "US Initial Post-Surrender Policy for Japan". The document set two main objectives for the occupation: (1) eliminating Japan's war potential and (2) turning Japan into a democratic-style nation with pro-United Nations orientation. Allied (primarily American) forces were set up to supervise the country, and "for eighty months following its surrender in 1945, Japan was at the mercy of an army of occupation, its people subject to foreign military control." At the head of the Occupation administration was General MacArthur, who was technically supposed to defer to an advisory council set up by the Allied powers, but in practice did not and did everything himself. As a result, this period was one of significant American influence, described near the end of the occupation in 1951 that "for six years the United States has had a free hand to experiment with Japan than any other country in Asia, or indeed in the entire world." Looking back to his work among the Japanese, MacArthur said, "Measured by the standards of modern civilization, they would be like a boy of twelve" compared to the maturity of the US and Germany, and had a good chance of putting away their troubled past.
On V-J Day, US President Harry Truman appointed General Douglas MacArthur as Supreme Commander for the Allied Powers (SCAP), to supervise the occupation of Japan. During the war, the Allied Powers had planned to divide Japan amongst themselves for the purposes of occupation, as was done for the occupation of Germany. Under the final plan, however, SCAP was given direct control over the main islands of Japan (Honshu, Hokkaido, Shikoku, and Kyushu) and the immediately surrounding islands, while outlying possessions were divided between the Allied Powers as follows:
It is unclear why the occupation plan was changed. Common theories include the increased power of the United States following development of the atomic bomb, Truman's greater distrust of the Soviet Union when compared with Roosevelt, and an increased desire to restrict Soviet influence in East Asia after the Yalta Conference.