Klaus Fuchs, the German - then English physicist who worked on the Manhattan project in the United States during the war and in the development of said bomb in the later 1940s passed on information to the Soviet Union which exploded their atomic bomb a few months before Truman went public with this and knowing the Fuchs had passed on hydrogen bomb information as well as our Uranium 235 output capacity (think how we measure Iran's threat), did so in order to push the project and sweep away any potential opposition in congress for the expenditures necessary. Fuchs' code name was "Rest" - and that is something I could never figure out.
Fuchs spent about 9 years in prison and died - I think fairly unnoticed - in 1988.
I knew a couple physicists in Russia in the 1980s. Smart guys with keen minds and I'm pretty sure they did work for the government in this area. Fuchs was something of a hot item at the time as he had just died and I mentioned something once at a cocktail party about him. My comment, whatever it was, was met with disdain and some rather stern expressions. They didn't like him because he had sold out his country - even though no money had changed hands of any import. "We would have figured it out anyway. He saved us time and money but we would have figured it out. YOU saved time and money by using him so what is the difference between you and us"?
Point noted.