Had the US not entered WWII, the Axis domination of all of the the Eastern Hemisphere would have allowed them to accumulate forces that could not have been successfully resisted by the US alone.
Unfortunately, many Latin Amerian regimes would have been only too glad to provide bases and assistance to Axis forces on this side of the oceans.
His remarks were not illogical at the time, but history showed them to be profoundly misguided.
Your right, what he failed to take into account was the American spirit. I think on 9/11 that spirit was awakened once more prompting me to think of phrases like "Waking the sleeping Giant" or "Disturbing the Eagles Nest". The people who decided to wage war on us are now going to find out that the American spirit knows no bounds. As President Bush said shortly after 9/11, you are either with us or against us.