Tattooed on Mr. Hegseth’s right biceps is the Latin phrase “Deus vult,” or “God wills it,” which he has described as a “battle cry” of the Crusades, the ruthless medieval wars where Christian warriors fought to take over Jerusalem from Muslim rule. Mr. Hegseth sees those battles as perhaps the most formative moment in the history of the free world.

In his book “American Crusade,” published in 2020, he describes the Crusades as “bloody” and “full of unspeakable tragedy,” but argues that they were justified because they saved a Christian Europe from the onslaught of Islam.

“Do you enjoy Western civilization? Freedom? Equal justice? Thank a crusader,” he writes in the book. “If not for the Crusades, there would have been no Protestant Reformation or Renaissance. There would be no Europe and no America.”