Complex sentence: After Julius Rosenberg told Ruth that the Manhattan Project was seeking to produce the first atomic bomb, David began to pass nuclear secrets to the Soviet Union via Julius from November 1944 until he left the military in 1946.
Split into simpler sentences: After Julius Rosenberg told Ruth that the Manhattan Project was seeking to produce the first atomic bomb, David began to pass nuclear secrets to the Soviet Union via Julius. Greenglass was a spy for about two years, from November 1944 until he left the military in 1946.