You can just read. The most surprising thing about WWII, at least to many Nazis, is that the US and Britain didn't join the Axis. German theories about race were largely drawn from British and American sources. The US fought WWII with a racially segregated military.
I personally think this puts some weight into the "race is a social construct" theory. That and the existence of mixed race people, and the stories of how they and their families have been arbitrarily treated based on varying perceptions. It's all pretty absurd.
The original collection of States were formed exactly to let groups divide. There were vastly different cultures, both culturally bonded, and also diverse, that were included from the earliest days.
source: anti-war, anti-slavery "whites" from that time
When Pearl Harbor happened, we [Roosevelt's advisors] were desperate. ... We were all in agony. The mood of the American people was obvious – they were determined that the Japanese had to be punished. We could have been forced to concentrate all our efforts on the Pacific, unable from then on to give more than purely peripheral help to Britain. It was truly astounding when Hitler declared war on us three days later. I cannot tell you our feelings of triumph. It was a totally irrational thing for him to do, and I think it saved Europe.
https://en.wikipedia.org/wiki/German_declaration_of_war_agai...
Regardless, Britain would not have supported a cohesive Europe-wide power even to fight the USSR, as proven by the postwar bickering with DeGaulle.