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

Only one country built it from “scratch”. The second (USSR) largely relied on information obtained from spies within the Manhattan project to create their bomb.