Assuming an "Isolationist US, no Lend-Lease, Britain (probably under weaker leadership) grudgingly sues for peace, USSR unlikely to drive the Nazis out any time soon, Hitler still declares war on America because Hitler is crazy" scenario - what are the USA's options for defeating Germany?
Is there any feasible non-UK route to invading mainland Europe, or is the USA forced to rely on war at sea and in the air (if even possible - bombers based out of Iceland and the Azores, perhaps?) until the Manhattan Project starts turning out bombs?