The U.S. was at war. Fear was everywhere. The Japanese attacked Pearl Harbor. Folks on the Pacific Coast feared a Japanese invasion. People were actually dissing the Chinese here in America because they thought they looked Japanese. Stories of Japanese atrocities were all over the news. I have some actual newspapers from that time. When I read them, it was like going back in time and experiencing the widespread fear of not only Japan but Hitler as existential threats to the 'home land.' People were sacrificing not only their male children to war but rationing had a big impact on what one could even buy and the overall quality of life.
To look back now and pass judgement from a modern perspective is, I believe, lacking much historical perspective and a bit arrogant. Hiroshima was an important military target. Nagasaki was an important port for the Japanese.