Historian says we’ve overlooked the fact that millions of Americans supported Hitler in the 1930s

These days, and especially since the deadly rally in Charlottesville, Va., last August, it has become clear to many Americans that the specter of Nazism in their country is not resigned to 1930s history. But until very recently, even that part of the story was less well known than it is today.

In fact, when Bradley W. Hart first started researching the history of Nazi sympathy in the United States a few years ago, he was largely driven by the absence of attention to the topic. Hart’s new book Hitler’s American Friends: The Third Reich’s Supporters in the United States argues that the threat of Nazism in the United States before World War II was greater than we generally remember today, and that those forces offer valuable lessons decades later — and not just because part of that story is the history of the “America First” idea, born of pre-WWII isolationism and later reborn as a slogan for now-President Donald Trump.

“There’s certainly a raw and visceral shock to seeing swastikas displayed in American streets,” Hart tells TIME. “But this is a topic I’d been working on for quite a while at that point, and while it wasn’t something I expected, it was a trend I’d been observing. I wasn’t terribly shocked but there’s still a visceral reaction when you see that kind of symbolism displayed in the 21st century.”