Sweden had an active eugenics program until 1975 and practiced compulsory sterilization until 2012. Eugenics was extremely fashionable across the Western world and only ended because of its association with the suddenly-unfashionable Nazis.

The U.S. had groups like the German-American Bund because the U.S. was and is a diverse country with freedom of speech. Other Allied countries had similar groups, such as the British Union of Fascists. Actual support for Hitler was a small, fringe minority.

On the other side of the scale, individual Americans had been traveling overseas to fight against the Axis powers for years, dating back to the Spanish Civil War when the 2,800 American volunteers of the Abraham Lincoln Brigade fought to defend the Spanish Republic. Also, the US supplied the Allies with $50 billion worth of food and supplies (over $650 billion in today's dollars), contributed militarily to the war, and spent another $17 billion ($160 billion in today's dollars) on the Marshall Plan to rebuild Europe afterwards. So, maybe let's take a look at everything in context and consider that, yes, individual people in a free country will sometimes support the wrong foreign leaders but "the US population" was not behind Hitler and repeatedly reelected governments that went above and beyond to contribute the nation's wealth and manpower to Hitler's defeat.