Although by ending the war, There was failure in diplomcy. 


By dropping the Atomic bomb there was a failure in diplomacy. 

Over 200,000 innocent civilians, were dead because of the detonation of both bonbs. 

"The Postdam Declaration which was  before  was worded so that the Empereror  Hirohito could be prosecuted as a war criminal.
  "We do not intend that the Japanese shall be enslaved as a race or destroyed as a nation, but stern justice shall be meted out to all war criminals, including those who have visited cruelties upon our prisoners. The Japanese Government shall remove all obstacles to the revival and strengthening of democratic tendencies among the Japanese people. Freedom of speech, of religion, and of thought, as well as respect for the fundamental human rights shall be established." (postdam Declaration)

This was something Japanese had to refuse because  the emperror was considered like a God like figure in  to the Japanese. . 
Eventally the Japanses had to agree to this, but that was only afer the atomic bombs were dropped and blow two cities two bits.  

Truman's actions showed no diplomacy when it came to making agreements with japan to end the war. 

It was obvious that we didn't give Japanese enough time  for japanese to accept surrender.