How were 2 countries able to discover and build nuclear weapons from scratch, simultaneously during the 1940s but most countries today can’t build them without decades of effort?

Question

How were 2 countries able to discover and build nuclear weapons from scratch, simultaneously during the 1940s but most countries today can’t build them without decades of effort?

In: Physics

Answer 1

I could build you a gun type weapon (little boy) in my garage. The hard part would be, and always has been getting enough highly enriched uranium and shaping it correctly. Plutonium weapons are much harder to make, but have a much higher destructive ratio, but Plutonium is even harder to make than enriched uranium. So besides getting your hands on the actual fiscion material, there’s the nuclear weapons non proliferation treaty, in which the world sanctions you if you do build a bomb.