It's a complicated history with a lot of things happening at once. Disentangling individual "variables" is difficult, and it is entirely possible that one cannot disentangle them (e.g., the reasons may have been multiple, which is common in complex matters of history). The thesis that the invasion of Manchuria caused them to surrender is usually referred to as the Hasegawa thesis. If you search for that in the /r/AskHistorians archives you'll find a lot of discussion about it. Here is something I wrote on it, from not too long ago, along with the thesis by Alperovitz that the US only used the bomb to scare the Soviets. If you are interested in a somewhat detailed timeline of events, this sort of functions as that.