The United States is the only country that has ever used nuclear weapons, and that was at Hiroshima and Nagasaki. There were no war crimes charges because it wasn’t a war crime. Truman made the decision to spare millions of lives that analysts predicted would have been lost if the US had invaded Japan instead of stopping the war by dropping the bombs. It was a military decision.
As for Nixon, he resigned 30 years after the nuclear bombs were dropped. His resignation came about because he planted listening devices in the rooms of his Democratic opponents during the election, allowing him to learn their strategies. He was caught and resigned before he could be impeached.
Many Americans are against nuclear weapons because we saw, quite vividly, what they do. At the time they were dropped the people involved didn’t know exactly what the effects of the radiation would be, and were horrified when they realized the long-term effects as well as the devastation they wrought.