You're making the assumption that you -need- the other side to sign an agreement. Japan was completely contained by the US at this point in the war.
This was a decision to hasten the end of the war, but did we ever actually leave? We still have bases in Japan. This was about hastening an end with a guarantee that the US would have full control of Japan.
Please don't misunderstand me though, Japan needed to be defeated thoroughly, it had acted and performed gross injustices against many people in the world. But these bombs were not necessary.
I will check out a summary and if it looks good I will give it a read. My googling makes it look a bit revisionist but I will check it out. Since we are starting a book club try 'With the Old Breed' by Eugene Sledge and/or 'Flyboys' by James Bradley to see the hell on earth the Japanese military had exported all over the Pacific. I was actually somewhat mentally exhausted by the end of 'With the Old Breed' and was glad when it was over. The book described something horrifying and grueling with no end in sight so I can't imagine how it would feel to actually have lived it.
James Clavell's fictional account based on his experiences in a Japanese prison camp is wonderful read too, 'King Rat'.
I talked with my grandfather often about his service as a submariner in the pacific, brutal, but oddly he never hated the Japanese. He saved that for the French.