Diplomatic Relations Between the Allies

This interactive “choose-your-own-adventure” activity traces the changes in U.S. diplomacy from 1931 to 1941 (requires Flash):
https://teachingamericanhistory.org/static/neh/interactives/neutrality/

“In World War II, the three great Allied powers—Great Britain, the United States, and the Soviet Union—formed a Grand Alliance that was the key to victory. But the alliance partners did not share common political aims, and did not always agree on how the war should be fought.”
Read more about the Big Three in this short article from the National WWII Museum:
https://www.nationalww2museum.org/war/articles/big-three

This short clip from CBS Evening News discusses the relationship between FDR and Winston Churchill: