The war being waged against the West also is a war against modernity. For nearly a thousand years, Islam reigned supreme in much of the world. But with the coming of the modern era — generally seen as beginning in the 18th century Christendom outpaced the Muslim world by almost every measure. “Those who know nothing of Islam pretend that Islam counsels against war,” the Ayatollah Ruhollah Khomeni wrote back in 1942.

Throughout most of history, war was seen as glorious — at least by kings, generals, and others who wielded power. No one expressed this view more eloquently than Genghis Khan, who conquered many Islamic lands in the 13th century, and who rhapsodized: “Man’s highest joy is victory: to conquer his enemies; to pursue them; to deprive them of their possessions; to make their beloved weep; to ride on their horses; and to embrace their wives and daughters.”

To modern people, such sentiments sound absurd. The terrible conflicts of the 19th and 20th centuries caused most Westerners to treasure peace, and to regard war as hellish, a last resort. In full....