James F. Byrnes

Byrnes became Secretary of State under Truman, succeeding  Edward Stettinius. In 1945, Roosevelt invited Byrnes, at that time still a Senator from South Carolina, to be part of the Yalta delegation (of which Hiss was a member) in an effort to include a conservative viewpoint. Byrnes later publicly opposed some of the Yalta agreements. 

In 1946, Byrnes advised Hiss to talk to the FBI about rumors that he was a secret communist. Hiss subsequently was interviewed by the FBI.