Amy Greenberg didn’t want to weigh in on Donald Trump. Ever since the president’s election, which she found stunning and demoralizing, the Penn State professor and historian of antebellum America has tried to keep her head down. She gives her lectures. She does her research. She works on her forthcoming biography of former first lady Sarah Childress Polk. But then, on Monday, Trump said, “People don’t ask that question, but why was there the Civil War?” He followed that up with a tweet claiming President Andrew Jackson, who died 16 years before the war started, “saw it coming” and would “never have let it happen!”

Greenberg couldn’t hold back. Prompted by an inquiry from CNN, she fired off a statement slamming Trump for “profound historical ignorance.” “Ask any fifth grader, ‘Why did the Civil War happen?’ That child can give you an answer,” Greenberg wrote on Tuesday. (Earlier in the morning, she’d queried her own fifth-grade daughter, just to make sure.) “How dare Donald Trump state that, ‘People don’t ask that question—but why was there a Civil War? Why could that one not have been worked out?’ Historians have been asking this since 1861.” Nor, she added, was there any evidence that slave-holding Jackson saw the conflict coming.

Such public criticism is new for Greenberg. “I haven’t critiqued a sitting president before,” she told me on Tuesday. “I’m a historian.” But now, precisely because she’s a historian, she says she’s “speaking out in favor of elected officials knowing basic, elementary level U.S. history.” “If we had an undergrad who wrote what Trump said in an essay, that student would not pass that exam,” she said. “That student would fail.”

Trump is hardly the first president to be wrong about America’s past. Bush flubbed certain facts, and Obama too. But Trump isn’t making minor mistakes. In barely more than 100 days, he’s taken historical ignorance to new depths in the White House. Now historians, a generally sober and restrained people, are rising up to defend historical facts in an age of “alternative” ones.

“I think historians, who tend to be kind of an insular group, need to take on much more of a public role and speak out about the past,” said Allan Lichtman, an American University professor who made headlines for predicting Trump’s election last fall, and who recently published a book making the case he’ll be impeached. “I think historians need to be fierce defenders of the truth.”