I'll make my previous lengthy ramble about logistics moot. By the end of June, 1940 we had exactly six army divisions in existence - and none of them armored. Mobilization didn't crank up until the Battle of Britain was underway. If the Germans had successfully landed soon after BoB, they would have had to wait about a year if they wanted to tangle with us. The 34th Division did it the quickest. A National Guard unit, it went from mobilization to the boat in eleven months, about six months quicker than any of the other early ones.
As it was, getting our forces mobilized, recruited, organized, trained (after a fashion), and shipped in time to participate in Torch in November, 1942 is an unappreciated miracle.
Lots of good points and they're accurate. Our support for Britain initially would most likely have been air, troop wise not much, we only had about 2000 or so in Iceland. But in 1940 the draft had been resumed in the US right at the same time Hitler planned to execute Operation Sealion, which was in September. Any total subjugation of Britain would have taken weeks or months, if at all giving us time to train troops and mobilize them.
It would have been difficult for us if Hitler did succeed in conquering Britain before we could prevent it, but I believe during that period we would have gradually built up our forces in Iceland and possibly sent troops to Ireland for plans to invade Britain. Either way, he would have faced us directly and economically the US was a giant compared to Germany, we would have beaten him with or without Britain.