It has been argued that the bombing of Hiroshima "could" be justified to some extent by it having shortened and ended ww2. However the bombing of Nagasaki was completely unnecessary the Japanese were already offering their surrender. The Second bomb was opposed by the US military, most of the government but was insisted upon by Johnson in order to "put the Russians in their place"
In order to minimizes the damage done, the American secret service lied to Johnson claiming that Nagasaki had much great cultural, military and political import to stop him from bombing Tokyo. fortunately Johnson was dumb enough to fall for it and basically used the second A bomb on a coastal tourist destination and fishing port, with half of the explosion effecting on the sea.

***Correction for President Johnson read President Truman