Leslie Groves was a brilliant engineer and manager.
The Russkies penetrated the Manhattan Project completely. Rhodes' 1994 sequel discusses this thoroughly. Of course, we executed the Rosenbergs for spying, but they were small time players in the whole affair. KIlaus Fuchs, Harry Gold, Ethyl Rosenberg's brother David Greenglass, his wife, and several others were involved.
The chief scientist for the Russians, Igor Kurchatov (alias "The Beard") was given access to all of the American data gathered by espionage, and promptly told his superiors that he could skip the American first step and immediately create a better and more powerful bomb; however, he was ordered to recreate the American Experiment, as it was called. Therefore, their first nuclear explosion was an exact copy of the Trinity device detonated in July of 1945.
After Russia's First Lightning in the summer of 1949, the decision was made to develop the H-bomb. The nuclear arms race was now in full swing.