Explain why roosevelt acted on the principle the enemy of my enemy is my friend and why churchill believed that if hitler invaded hell he would work with the devil himself

Roosevelt felt that despite Russia being communist, Hitler posed a more immediate threat, and that before they get at each other, Hitler had to be knocked down. Churchill believed that Hitler would do anything to get what he wants, albeit killing millions or sell his soul to the devil himself. Hitlee also openly voiced his idea of fascism everywhere, which didnt exactly comfort churchill