Churchill was undercut by FDR in the Tehran and Yalta talks. FDR thought he could charm Stalin, and Stalin played the ailing FDR like a fiddle. There is a really good book by Lynn Olson and Stanley Cloud on the contributions of the Polish Squadrons fighting under RAF command in the Battle of Britain called “Question of Honor, Kosciusko Squadron.” It gets into to the betrayal of the Poles, largely orchestrated by FDR.
With the help of communist spies within FDR's administration, Alger Hiss comes to mind.