Guys, it's not about having the scientists, it's about having the resources. America, as the richest country and the greatest industrial power, could afford to pour millions into an untested blue-skies project while still being at war. (Remember that it would have been a lot easier for the Soviets to do it later, because their spies meant that they knew a lot of what the Americans had had to find out expensively through trial and error).
If we take say 1930 as our POD date (to allow non-Nazi Germany with Jewish scientists staying there, and so forth) there are only three countries that could have invented the bomb IMO.
- USA
- Germany
- Britain (but this is iffy, and only if it was a joint project with all the dominions).
France, Italy and the USSR could have duplicated the bomb programme once someone else had worked out all the knots (as indeed the USSR did in OTL) but they lacked either the resources or the political will to utilise a vast amount of them on a dodgy project. Remember, after all, this was the era of the Depression.