OK, the question that you ask is confusing to me. Do you mean would the US attack Japan if they where not at war?
If you mean that was it really unavoidable if the US was at war with Japan then yes and no.
If Japan gave up then no bomb.
If japan did not then realistically no.
Andrew J. Rotter in Hiroshima: The World Bomb looked at a lot of primary sources and came to the conclusion that there was 3 things that made it inevitable.
Its a weapon that was made to be used. And was. In other words The US did not spend BILLIONS of dollars, create 2 cities from scratch , and hire 100,000's of people to NOT use the bomb. Like every other weapon before, from crossbows to Napalm to mustard gas they where made to be used in war and where. Jokingly As they said in Crimson Tide, a movie about nuclear war, You don't put a condom on if you are not going to fuck.
Truman did not have the "cache" to stop it. Truman was in office less then a few months and he did not have the political power to realistically just say wrap it up boys this is not going to happen. This was what FDR wanted and it would be done.
The ball was rolling. This ties the last two together. From the construction to the planning to the execution it was just assumed, well yeah this is what we are doing. Not to say that there was NO discussion of alternate targets but it was a very small minority.