You must remember, that speed and surprise were needed to defeat the Japanese. American intelligence from the period was, to put it shortly, rubbish.
Japanese were far expanding into China and it was a threat to American forces fighting there. They were in territory they knew and understood. And for America it would have been a loosing battle.
Also we know from Pearl Harbour that, not only were the Japanese prepared to use obscene tactics to achieve their goal. But also because it is a country based on honour and pride, they were willing to do anything to see victory.
But there were also, other reason for using atomic warfare. This is fundamentally to do with the arms race between the USA and the former USSR. Americans were keen to prove themselves victorious over the Communist regime of the USSR by being the first to successfully detonate nuclear weaponry.
Therefore you actually see the emergence of two independent reasons as to why nuclear force was used against Japan.
History lesson aside, nuclear warfare is a disgusting practice which is designed to cause mass destruction quickly at the expense of thousands of innocent lives. So I'm totally against it.