It was following the proclamation by Albert Einstein and Robert Oppenheimer that a nuclear weapon could not be built that the United States of America fully came to realize the possibility of an invasion of mainland Japan. Imperial Japan, now weak and nearly destroyed, but not kneeling, would not surrender despite U.S. demands and bombing of key cities.
Emperor Hirohito famously stated in September 1945 that the Japanese people would never surrender and that an Axis victory was still withing possibility. It was under these circumstances that the United States and the USSR came to an agreement about an Allied invasion of Japan. However, when it became apparent that Joseph Stalin had his sights set on a Communist Japan, the U.S. was forced to launch an early invasion or face the consequences.
American forces landed on the Japanese islands through several points, including Kyoto itself. The fall of the Imperial city was a symbol of the defeat of Japan, but the Japanese still refused to give in. By February 1946, Operation Downfall was in effect, but the casualties were considered far too high. As American troops seemed on the verge of completely occupying Japan and securing victory for the Allies, the Soviets landed a force under Marshal Zhukov to hit the northern end of the Honshu.
The Soviets captured the relatively poorly defended regions with ease, the first easy victory for them since the 1941 betrayal by Hitler. Finally, Japan's military machine that had ruled East Asia collapsed, and the U.S. deposed Hirohito, Tojo, and their government along with their supporters in the military.
Like Vietnam, Korea and Germany, Japan was doomed to be split into the feuding groups which had torn the other nations apart. The Soviets established a dictatorship over North Japan and South Japan was ruled by a republic with strong Western influence. The border line became an increasingly tense topic between Eastern Bloc and Western Bloc leaders, as the agreements between the military leaders was extremely vague about where on Honshu the dividing line was. This caused border tensions, contributing in part to the Cold War of 1946-1965. In 1949 a dematerialized zone was set up to divide the two nations from destroying the islands, but this proved to be of little use in the Japan War and World War 3.
It was following a declaration that part of the dematerialized zone belonged to the North that tensions started heating up in 1951. The South moved troops to the edge of the dematerialized zone, and, claiming to be acting in self defense, a North Japanese artillery brigade opened fire. Immediately battles broke out between the two sides, and the U.S. found a controversial topic in its internal politics. There was much debate in Congress over whether it was all right to fight two wars, the Korean War and the Japan War, at once, and the result was a compromise. The U.S. and U.S.S.R. agreed that neither would intervene if the other didn't. It was a rare moment of co-operation. The North Japanese forces were outdated and smaller. The warmongering government there had decided in early 1951 to invade, and little planning appears to have gone on since then. In 1952, the Battle of Takasi proved a major turning point for the Southern Japanese soldiers, who had proved only to be able to win relatively inconclusive battles until that point. The North Japanese took 36,000 casualties out of 70,000 men and the South Japanese took only 4600 casualties out of 90,000 men. The sheer losses drove the relatively small North Japanese back, and in 1953 the North Japanese agreed to pay war reparations. They would never forget, and did not forgive. The Japan War was remembered as the world moved towards another World War.