One of the things that spurred the US scientists was intelligence that they were in a race with the Germans. When the war ended, they were stunned to see how far the Germans were from anything useful, barely past the theoretical stuff. And the Japanese were way behind the Germans. Some of them felt they’d been lied to.
Well until you actually took over Japan and Germany and looked at their labs and papers, you couldn’t be sure they didn’t have a valid atomic bomb program.
If we could do it, there was no reason, given enough time, they couldn’t do it. You couldn’t take the chance of discounting it.