Obviously geography plays a role in war. But to think that large oceans offer us the same protections they did in the 18th century is a bit naïve, at least.
Rather than cherry pick one or two little aspects of a conflict, why don't you simply address directly and fully how well you think a bunch of disjoint tribes of Angle-Saxons or Celts or Scots could have repulsed the Nazi war machine. Even the Romans managed to overcome the challenges of channel. True, the Celts/Scots made life difficult enough in the Highlands that the Romans decided Hadrian's wall to keep the barbarians out was better than trying to conquer them. The Romans did not have the Luftwaffe, Panzers, nor other modern weapons.
Lacking the end of the war, but facing real resistance that conventional weapons couldn't break, do you think the Nazis would not have eventually developed the atomic bomb? Or perfected their rockets?
It cost the US some $23 Billion in current dollars to develop the A-bomb. That is $75 for every man, woman, and child in our nation today, probably double that to $150 for our population in the 1940s. Or, something like $700 for every household. The A-bomb is horrible thing. The only thing worse than having it, is not having it when violent and avowed enemies do. How does a non-nation, with highly desirable land full of rich natural resources, develop the weapons needed to defend against nations with active military programs?
The American Indians were unable to defend against the English war machine of the 17th and 18th centuries including muskets and cannon, and number of recruits. They were ultimately unable to defend against the US government.
Without government, how do the peoples of the lands now called the USA defend against the Chinese, Soviets, or even Mexican governments if any of these decide they'd like to expand their holdings? Even more challenging, lacking government how do the peoples of Texas, Alaska, and the Dakotas prevent Arab nations from bombing their oil wells so as to reduce supplies and increase the value of Middle East oil?