Hiroshima might have been necessary but Nagasaki was definitely unnecessary. To prove that the first atomic bomb was not a one time trick, the US could have easily bombed some non populated area for the second time instead of massacring hundreds of thousands of innocent civilians.

Be very careful to consider the situation and weapon in light of the times not from x decades on. This was a new weapon that promised efficiency - one plane could do what 100-200 and the right conditions could do. The firebombing of Tokyo that destroyed 16sq mi of the city was more severe and the us had the will to keep doing it.

From the us leadership point of view, testing their investment, avoiding an invasion’s casualties, avoiding the blockade and avoiding soviet success had their advantages. There was no reservoir of goodwill left in the pacific war. And it drew a line in the sand for postwar politics which, of course, could not last. And they knew that war fatigue was present in the us.