It was the proof of concept for the following development of Nuclear weapons. Even though the war with Germany was already over, the war with Japan was not. Even though many Generals and even Oppenheimer were opposed to using it against Japan on Hiroshima and Nagasaki as all agreed the outcome of the war would have been the same. But Truman pushed hard (he said the genie is out of the bottle and putting it back is not possible, and since we have such a powerful weapon it would not make sense NOT to use it.) and hoped it would show the world and specifically the USSR that the USA had the mightiest weapon. In effect, it had a contrary effect and the USSR had the successfully test their first nuclear bomb in 1949. The History channel is airing documentaries about those events regularly.