What do you mean by "a threat to the US"? Japan was never in a position to, say, existentially threaten the United States. But it was in a position to threaten US allies and holdings overseas, and to essentially "raid" the US in various ways (you could see Pearl Harbor as something like this).

The reason the US went to war against Japan was not because anyone feared the Japanese taking over the US. They could not do that at any stage.

If you are asking, were the Japanese essentially beaten prior to the atomic bombs being dropped? The answer is: yes, essentially, though they still had some overseas holdings. But the US demand was not that Japan be defeated militarily, but that they unconditionally surrender. They had not done that at that point, and did not look like they were going to do that.