In short, America needed a Japan that was rebuilt and a friendly state as quickly as possible in the immediate aftermath of the Second World War. This was in order to have a stable, undivided platform from which to counter what many high-ranking military experts understood was going to be the Third World War right on the Second’s heels: the looming confrontation between the Soviet Union and the Western Powers.
As per the Yalta and Potsdam Conferences, though Japan had studiously (and much to Hitler’s dismay and ultimate downfall) refrained from declaring war on the USSR, Stalin had agreed with Churchill and Roosevelt to declare war on the Japanese Empire and invade, first their colonies in Manchuria and Korea, and then the home islands. What that meant in terms of the Western postwar outlook would be much the same as what was already shaping up in Europe: the division of such states into Soviet and Western blocs. Such a state of affairs was wholly undesirable to the US, who looking beyond 1945 saw the policy on containment of Soviet influence to be its top priority.
As late as August 10th, 1945 – after the atomic bombing of Hiroshima but before the destruction of Nagasaki – the Japanese government was still sending its request to conditionally surrender to the Allies, with their one and only condition: “that the stipulations of [the Potsdam Declaration] do not include any demand for alteration of the authority of the Emperor to rule the state.” Such a precondition were deemed unacceptable by both the US and the USSR, with Commissar of Foreign Affairs Molotov being described as “skeptical,” and thus the war carried on until September 2nd when unconditional surrender was at last accepted.
There were trials, and members of the Japanese military and civilian government were tried and between the IMTFE and the prosecutions conducted by independent nations, according to Dower some 5,700 people were convicted of Class A, B, and/or C war crimes, resulting in almost 1,000 death sentences, nearly 500 life sentences, and more than 2,900 lesser sentences. Yet some of the worst criminals, it is true, did essentially get away scot-free, most notably the commanders of the infamous Unit 731, which had been given immunity from prosecution for their human experimentation and biological weapons research in exchange for that research, and at least as important, denying the Soviets access to that research (which would prove at least partially futile).
Crucially, though (and ironically considering that the Allies had notably turned down requests to surrender on this precise basis), the Japanese monarchy and government was left largely institutionally intact. Unlike West Germany, there was no corollary to “de-Nazification” in Japan. Yes, there were purges of the extreme ultranationalists and right-wingers, but they were often allowed back ‘into the fold’ rather quickly. Why? Because especially after the shocking fall of the Republic of China to the Maoists in 1949, it seemed to most that Asia was on the cusp of toppling like dominos to communist influence. The US determined at a fundamental policy level that a stable, united, friendly, and anti-Communist Japan was absolute priority. That meant people with a proven track record of industrial, business, and managerial success – lo and behold: it was often the very same rightists.
The Emperor was declared by MacArthur and Co. to be not responsible for what had happened over the course of the war, with the blame instead falling squarely on the military brass like Tojo. Symbolically, this was significant because it allowed the people as a whole to say, in essence, that the militarists had taken over and done bad stuff, but that was them, not us, and they were already held responsible.
Though there are elements of the modern Japanese right-wing that do out-and-out cling to war crimes denialism, they are an extreme (albeit vocal) minority. Far more significant (and I don’t intend to downplay its corrosiveness) is not denialism, but rather the tendency to downplay the war crimes. What is known in China, for instance, as the Nanjing Massacre (南京大屠杀) is often known in Japan as the Nanjing Incident (南京事件) which – you can no doubt see – is a significant difference of terminology. Nevertheless, in spite of the fringe deniers and the broader public’s propensity to not think much about such incidents, it must be said that the Japanese government has issued repeated and formal apologies for its atrocities and war crimes, notwithstanding the fact that other counties involved like to frequently announce in a fit of nationalism that it wasn’t said enough, or sincerely enough, or with enough conviction, etc.
Dower, John. (1999) Embracing Defeat: Japan in the Wake of World War II.
Translation of intercepted Japanese messages, circa 10 August 10, 1945, Top Secret Ultra
Memorandum of Conversation, "Japanese Surrender Negotiations," August 10, 1945, Top Secret