What many have said is that an alternative to Operation Downfall was to just continue the close blockade. At the very least it would have killed lots of the Japanese who would have attempted resist a very much delayed invasion, reducing casualties by a lot. This choice was not popular with the Army or with the politicians. The politicians didn't like it because the public was getting real tired of war and wanted it over. Those same people said the casualties were being greatly over hyped. The Army didn't like it because the Navy would get all the credit while the Army had to keep all it's troops in case it didn't work.
IMO the argument that the bombs were dropped as a statement to over-awe Russia is a retrospective analysis based on what we know now. Truman and the other decision makers didn't know there was going to be a "Cold War". They weren't thinking much about keeping Russia out of Japan. Some people were, but not many. Things were starting to get tense with the USSR but weren't there yet. On top of that, the US Army did not predict the rapid advance Russia made into Japanese territory when they entered the war. If Russia had already made such advances when the decision to drop the bomb(s) were initially made, it would make sense we were trying to scare them. That was not the case. Once the initial decision to drop any bombs was made, the idea of stopping it at the last minute wasn't realistic. Military organizations don't work that way. Too much momentum. The AAF would have drug their heels at every step.
IMO the argument that the bombs were dropped as a statement to over-awe Russia is a retrospective analysis based on what we know now. Truman and the other decision makers didn't know there was going to be a "Cold War". They weren't thinking much about keeping Russia out of Japan. Some people were, but not many. Things were starting to get tense with the USSR but weren't there yet. On top of that, the US Army did not predict the rapid advance Russia made into Japanese territory when they entered the war. If Russia had already made such advances when the decision to drop the bomb(s) were initially made, it would make sense we were trying to scare them. That was not the case. Once the initial decision to drop any bombs was made, the idea of stopping it at the last minute wasn't realistic. Military organizations don't work that way. Too much momentum. The AAF would have drug their heels at every step.