Final answer:
Churchill urged Roosevelt to consider the risks of a successful German invasion of Britain and the implications for the US, while Roosevelt, being aware of these concerns, maintained US neutrality by providing military aid without directly entering the war until later.
Step-by-step explanation:
Winston Churchill was acutely aware of the dangers posed by Nazi Germany, and he communicated his dire warnings to President Franklin D. Roosevelt, emphasizing that a successful German invasion of Britain would put the United States at risk. Churchill hoped that the US would join the Allies, but Roosevelt was more cautious, focusing on support that wouldn't directly involve entering the war, such as supplying munitions and engaging in diplomatic strategies like the Atlantic Charter. Roosevelt prioritized giving Britain the means to defend itself while maintaining official neutrality, which reflected the American public's opposition to entering the war in 1940. Despite this, Roosevelt's actions, like the destroyer-for-bases deal and the peacetime draft, were preparing the US for a less neutral role. Throughout the war, both leaders maintained close cooperation, with Churchill successfully lobbying Roosevelt at key moments, such as prioritizing the invasion of Sicily over France and agreeing on an 'unconditional surrender' doctrine at the Casablanca Conference. This close wartime diplomacy continued throughout the conflict and laid the groundwork for the post-war world order and the establishment of the United Nations.