The USA did not overrun Europe or the South Pacific or Asia. Nor come up with a Final Solution. The German's were the architects of World War II, the Japanese willing partners. The Second world war started officially, one could say, in September 1939, but Japan had already invaded China in 1937 The USA did not even enter the War until Late 1941, close to two years after it started and only after Japan launched the offensive in Pearl Harbor.
So yes. the answer to your question would be: If Germany or Japan had dropped an atomic bomb it would have been a crime agaainst humanity, one in a long list of crimes already committed before the United States entered the war and helped the Allies bring about an end. Thank goodness for Generals like Ike Eisenhower and US soldiers, who, unlike their British Allies learned to adapt and overcome in the face of adversity. Thank goodness for the Aussies and the Canadians.
And as a final note. Thank Goodness that the Nazi's never achieved their goal of building an A Bomb, which they were most assuredly seeking to do, because they would not have stopped after two. Even if we had surrendered.