Just after last week’s terrorist attack in Barcelona, a pro-Islamic State website posted video from the scene along with a message in Arabic saying, “Terror is filling the hearts of the Crusader in the Land of Andalusia.” Let’s unpack that. “Crusader” is a term jihadists use, pejoratively, for Christians. More specifically, of course, it refers to the Christian soldiers who fought a series of wars, beginning in 1095, to recover Jerusalem and other parts of the Holy Land from the Muslim armies that had burst out of Arabia four centuries earlier. Andalusia indicates the territories of the Iberian Peninsula that...