History, however, is best read within the lines, not between them. Ullrich’s biography has hundreds of pages of lines, and many of those lines don’t bring Trump to mind. Imagine if, for example, Kakutani had also highlighted Hitler’s military service in a world war, his limited romantic involvement with women, his participation in a violent coup and resulting imprisonment in the 1920s, or the virulently anti-Semitic views he expressed before coming to power in Germany. Readers might not have grasped the implicit message of Kakutani’s article (assuming Kakutani intended her review as an allusion to Trump, which she has neither confirmed nor denied). The signal would have been scrambled.

Ullrich, for his part, had to read Kakutani’s review twice before concluding that it “was intended to do something more than just praise my book.” He doesn’t agree with the subtext that many people spotted.

“I don’t think it’s appropriate to compare Donald Trump with Hitler,” Ullrich told me by email through his translator, Jefferson Chase. “In my opinion, it significantly trivializes Hitler. And as a rule Hitler comparisons are not about fairness. It doesn’t matter whether they’re historically valid or not. They have a political purpose. Hitler is considered the embodiment of evil, so when someone is compared to him, it’s the same thing as rendering a devastating judgment about that person.”

Yet several modern-day political figures have been so judged. Trump’s rival in the U.S. presidential race, Hillary Clinton, once likened Russian President Vladimir Putin’s aggressive tactics to Hitler’s. More recently, Philippine President Rodrigo Duterte responded to claims that he was acting like Hitler in waging a bloody war against drugs by … reinforcing the comparison. Hitler murdered millions of Jews, Duterte noted. “Now, there are 3 million drug addicts [in the Philippines]. I’d be happy to slaughter them.”

In fact, we’re living through a period that is ripe for Hitler comparisons. According to the political scientists Ronald Inglehart and Pippa Norris, “populist authoritarian leaders”—defined as nativist, nationalist, anti-establishment politicians who emphasize the “personal power exerted by strong and charismatic leadership which is thought to reflect the will of the people”—are gaining popularity in democracies around the world. Inglehart and Norris explain the trend as a backlash against transformative social and cultural changes. Other scholars cite factors such as dissatisfaction with dysfunctional politics, economic inequality worsened by globalization and financial crises, and concerns about perceived increases in immigration, crime, and terrorism.

The political scientists Barbara Geddes, Joseph Wright, and Erica Frantz, meanwhile, have found that “personalist dictatorships”—where one person has control over policy, personnel, and security—have grown more numerous since the end of the Cold War. Along with governments dominated by one political party, they are currently the most common form of autocratic regime.