American Involvement in a U.K.-less WWII

So, let's say that, using whatever POD you find the most plausible to accomplish it (no Churchill, disaster at Dunkirk, some combinations of factors), let's say that the United Kingdom sues for peace with Nazi Germany at some point in 1940. The Germans won't be occupying Britain or anything like that, but at least in the near term, the two countries won't be fighting. Now, I imagine that this would have any number of huge effects for the war in the Mediterranean, the Balkans, and for the planned invasion of the Soviet Union, but right now, I'm curious to know how this will effect America's involvement in the war in Europe.

Peace being declared on the Western front in Europe could deeply affect the American domestic political situation (FDR may not run again, or he may lose, perhaps to a different Republican nominee), but the tensions between the United States and Japan will still be there, and it is very likely that war will still break out between the two powers before the end of 1941. Hitler, still given to boneheaded decisions, would likely still declare war on the United States in such a scenario.

However, in a world where the United Kingdom is at least nominally neutral, how would the Americans prosecute their war against the Nazis? Would there be more emphasis on trying to invade German-occupied Europe through the soft underbelly of Southern Europe/the Balkans? How would America's aims in Europe be different in such a scenario?

Or would Britain be liable to resume hostilities against the Germans if they declare war on the Americans? If so, how much difference would it make in the final outcome if there was a year to a year and a half where Britain wasn't fighting the war?
 
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