Kevin M. Kruse is swamped. Hunched over his desk on a Wednesday in October, fresh off a lecture that morning, the Princeton University history professor needs to prep for a dissertation defense. He has letters of recommendation to read and emails to answer. Ungraded essays sit in fat stacks on his desk.

Like his colleagues, Kruse balances many responsibilities. But there’s one he’s been neglecting all morning: He hasn’t checked Twitter.

He logs on to shocking news: Pipe bombs have been mailed to Democratic leaders and prominent critics of President Trump, in what appears to be a coordinated series of assassination attempts. Like with so many news stories, people are wondering what it means. Kruse has work to do.

If you’ve heard of Kruse, it’s probably because you’ve read his tweets. Online, the historian specializes in serialized posts, called threads, that lend historical context to breaking news or skewer a version of history spouted by right-wing agitators. Yes, there’s precedentfor athletes protesting during the national anthem, he wrote when Colin Kaepernick made national news. No, Abraham Lincolnwouldn’t like Trump, he said in another.

In this era of political crisis, people turn to social media, where pundits are waiting to reassure them that history is on their side. But most of them haven’t studied history as carefully as Kruse. The Princeton professor and his like-minded peers don’t think they can afford to confine their knowledge to classrooms, journals, and the occasional book. Some are wading into the more rambunctious realm of Twitter, and it takes special skills to land punches and avoid getting chased back to the ivory tower. Kruse is the best at it, and he’s showing others the way.

It’s intense. He’ll leave for class, and come back with thousands of notifications from strangers, some of them demanding: You have to correct this. Please, we need you. He has a hard time staying off his phone. There is always an emergency — tons of people saying something ignorant or cherry-picking half-truths about American history, and only so many experts like Kruse who are qualified to offer a swift, accurate rebuttal.

Historians like Kruse are staring down a challenge: How do we cope with being this relevant?...