Harry Dexter White was one of the top Soviet agents of influence in America. This man steered Yalta protocol on two million East European refugees which were handed over to Joe Stalin and murdered or sent to Siberia. He passed documents to the Soviets and blabbed at parties to his counterparts. The FBI warned Truman of White before his appointment to the IMF; Truman ignored it.

"Stalin's Secret Agents" and "Blacklisted by History" by M. Stanton Evans are the best places to find out more.