His research and teaching focus on religion, violence, nostalgia, and apocalypse, whether manifested in the European Middle Ages or modern world.

This includes events and ideas such as the Crusades, the so-called apocalyptic “Terrors of the Year 1000,” as well as modern medievalism, such as recent white supremacist appropriations of the Middle Ages and pop culture phenomena using the some vision of the medieval world (such as Game of Thrones or various video games).

He has published several books and numerous articles. He has also been an expert speaker at Harvard, Princeton, Georgetown, and the University of California-Berkeley in the United States (among others), and has also presented his research at universities in England, Scotland, Germany, France, and the Netherlands. In 2010, he was a visiting researcher at Westfälische Wilhelms Üniversität-Münster, and in 2023, he was the Donald J. Bullough Fellow in Mediaeval History at the University of St. Andrews, UK.

His public writing has appeared in such places as The Washington Post, Time, CNN, and The Daily Beast.. He was between 2018-19 a columnist for Forbes, and was a columnist for Smithsonian Magazine between 2020-21. Interviews with him have aired locally, nationally, and internationally. He has appeared as an expert on several shows on the History Channel.

With David M. Perry, he co-authored: The Bright Ages: A New History of Medieval Europe (Harper Books, 2021). He and David have teamed up again on their latest book: Oathbreakers: The War of Brothers that Shattered an Empire and Made Medieval Europe (Harper Books, 2024).

See his academic CV.