That is impossible to surmise.
I mean there are two main arguments that people make, the bombings of Hiroshima and Nagasaki were war crimes, and that they were completely justified.
Either one is arguable, the justifiable one has pretty solid data as to how many people were being killed in American bombing raids and how many people would have died in a land invasion of Japan. https://www.history.navy.mil/about-us/leadership/director/di...
I mean it is pretty much the opposite of impossible to surmise! Given that there are lots of well known arguments on how the use of the atomic bombs were justified because of lives saved I find it a bit much to claim it was arguably one of the most barbaric acts of the war without even a nod towards hey, maybe it wasn't though, since the phrasing implies that hey everyone everywhere agrees it was pretty darn barbaric.
- Most cities in Japan had already been extensively fire-bombed - The Japanese High Command were not shocked that atomic bombs could exist or that yet another city was obliterated - The Japanese - Soviet war was also not going well for Japan. They would much rather surrender to the Americans than the Russians.
You are framing it as if a land invasion was inevitably the only other option. That's also impossible to surmise.
Something that is evidently not known here is the meaning of surmise, which my dictionary tells me is
"suppose that something is true without having evidence to confirm it."
so, I think what people are trying to say is it's impossible to know for certain. And yes, it is impossible to know for certain if a land invasion would have happened, but it does seem like a pretty good chance of it given the planning for one.