Wrong!
right up to Pearl Harbor there was a significant debate in the US about which side in europe was 'right' and what we should or should not do about it.
There was ZERO consensus right up until germany's ally bombed our obsolete navy, in a semi-colonial anchorage, and pissed off most of the American public.
Oh, and it took half of a year before we were even able to assume command of the AVG, there were no troops standing ready to invade anything, we'd have taken months to provide support to the Brits in any event..."before Hitler could blink..." does not even equate.
Of course behind the scenes we were providing lend-lease support (practically illegal in constitutional terms) to Russia and England. We were interdicting Japanese shipping, which was in a very real sense an act of war. Plenty of Americans understood what was coming and what we'd have to do about it. Much as has been the case today, the isolationists and pacifists were slow learners.
You say that as if it was fact, it's all a matter of speculation. In my opinion if Britain had been invaded the US would have significantly stepped up it's presence in the North Atlantic, perhaps sending large numbers of troops to either Iceland or Ireland or both. I'm not debating the pacifist movement existed in the US, only that the Roosevelt administration, for all its faults clearly understood the dangers of Hitler and fascism and would have taken appropriate steps to deal with it.