The US was justified in dropping the atomic bombs on Hiroshima and Nagasaki.
Japan already killed over 2,000 people in the Pearl Harbor bombings. We had done nothing to provoke them, unless you count our cutting off trade with Japan, limiting their oil supply. That was also reasonable, as they were rapidly conquering the Indian Ocean and wanted to take the Pacific too.
In addition, this was nearing the end of WWII. Hitler and Germany were already out of the picture, but Japan was still strong, holding several hundred islands in the Indian Ocean. We moved to take out Japan by defeating the Japanese on several islands, but because of their kamikazes, we took heavy casualties, and the Japanese lost far less men than we did. A full-scale invasion of Japan would have cost thousands upon thousands of American lives, and that was simply unacceptable! Dropping the atomic bombs saved American lives and crippled the Japanese from attacking us, thus effectively ending WWII in the Pacific Ocean.