In the simplest terms?
They didn’t want a repeat of the end of the first world war which only gave them a 20 year respite.
When they stripped the Kaiser through the intervening years they ended up with hitler so they thought to themselves well might not be the best idea to do that again. Macarthur played a huge roll and said “Destroy him and the nation will disintegrate” in a series of documents from the US state department documents released in 1977.
He went on to say “His connection with affairs of state up to the time of the end of the war was largely ministerial and automatically responsible to the advice of his counselors,” and “I believe all hope of introducing modern democratic methods would disappear and that when military control finally ceased some form of intense regimentation probably along Communistic lines would arise from the mutilated masses.” so the emperor had a powerful advocate in the US military. They believed they needed him so he was to be kept.
That the emperor surrendered and issued the statement himself instead of forcing an invasion of Japan was a huge factor in his favor. As they surrendered the Americans didn’t have to deal with the anger of their population on the heavy death toll for the invasion. Interesting tidbit all purple hearts issued since ww2 are surplus from the aborted invasion of Japan they were expecting mass casualties and injuries.
The fact that there was a coup attempt to stop the surrender and it failed gave the emperor some breathing room as this reinforced the idea that he was simply rubber stamping the will of the military.
It was rapidly becoming apparent to the allies that they would have to deal with the Soviet Union.
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