What were the strongest arguments for using the atomic bomb against Japan to end the war in the Pacific?

There were two strong aguments for using the Atomic Bombs on Japan. Firstly it was anticipated, quite correctly, that it would bring the war to an early close with dramatic reductions in USA military casualties and costs - an argument bound to be popular with the American public and, therefore, popular with its politicians.

The second reason was R and D (research and development. Having built the bomb and conducted the Trinity test, the USA needed to know how effective it was in terms of ground damage and casualty rates. The USA knew the war would eventually come to an end, bomb or no bomb, and they wanted to come out of it as undisputed world leader. Telling the world they were able to use the bomb gave them a big advantage.