is this film pro-Japan anti-American?

As Robert McNamara (who was involved in the firebombing of Tokyo, in which even more people died than in Hiroshima and Nagasaki)said in the Fog of War, had the US lost the war, they would have been tried for war crimes. Had I been Truman I would probably have made the same decision, and I suppose the true horror of what the atomic bomb could do could only have been realised after it was used, but people were hung at Nuremburg for less than what happened at Hiroshima and Nagasaki.

Imagine if Japan or Germany had invented and used the atomic bomb first, but at a point too late for it to win them the war. Those who ordered its use would certainly have been executed and the use of Nuclear Weapons outlawed as a crime against humanity. I doubt the US would have been as forgiving as Japan has been.

The Japanese government was not 'under' Emperor Hirohito in anything other than a very ceremonial sense. Hirohito was not the decision maker in the conduct of the war - that was Prime Minister Tojo, who was essentially responsible for the entire duration of the conflict.

People take Hiroshima and Nagasaki completely out of the correct historical context - civilian populations had been targeted freely since the very start of the war. Since before that if consider the bombing of Guernica in the Spanish Civil War by the German 'Kondor Legion'. In reality every major participant in World War II targeted their enemies civilian populations. The Germans bombed English and Russian cities, the English bombed German cities, the Americans bombed German and Japanese cities and the Japanese targeted civilian populations in China.

The Atomic bombings were undertaken as a less bloody way of ending the Pacific Campaign. The alternative was a massive conventional invasion of the Japanese mainland. The casualty projections of that were in the millions - and the brunt would have been borne by Japanese civilians.