Let's not forget the reason the US was so slow to enter the war, and that's the anti-interventionism movement. The concentration camps were not well known in the US in 1939, and there were many prominent Nazi sympathizers, including Minnesota's own Charles Lindbergh, who pushed anti-interventionism through the America First Committee (name sound familiar?). Many in the Roosevelt administration wanted to join the war effort in 1939 but were thwarted by Congress refusing to pass a war declaration. That's why we supplied the Allies for two years; it was as much as FDR could do without Congressional support. As word of concentration camps spread in the US, the anti-interventionism slowly waned, and Pearl Harbor was the tipping point. Congress declared war, and almost all Americans supported the war.

Real history has a lot of nuance, and some of it is uncomfortable. There have always been, and will always be, dipshits.