The Nazis developed the theory, but due to problems with materials science couldn't actually manufacture the stuff. After the war when Paperclip was enacted, the researchers finally gained access to things they needed to make their designs reality along with Reich (a Jew that had fled Germany right before the War) connecting with Ruehl a Paperclip scientist. They were the pair that made the power breakthrough. Reich did not know that Dulce was a Nazi run operation in fact most of the researchers there didn't know.