“Woe to the Assyrian, he is the rod and the staff of my anger,
and my indignation is in their hands. I will send him to a deceitful nation . . . ”
—Isaiah 10:5-6, Douay-Rheims
Confronted by the rise of insurgent Islam and the political reality of jihad, many Christians, eager to formulate a response, have turned to the Crusades. Can the rationale for the Crusades be transferred to, or imposed on, the “War on Terror”?
In order to understand the theological case for the Crusades, we must carefully attend to the notion of theological argumentation that obtained in the Middle Ages, setting aside current styles. The perspective of the historian is necessarily ruled out, even if he may discover or collate or provide the texts from which the theological rationes are to be culled by the theologian. An approach that makes use of a posteriori evidence of what is often abusively called “popular piety” or “ideology” or “propaganda”—or, if we are of a more traditional bent, of a certain positivistic romanticism about Europe and the Christian Faith—will not yield an understanding of what for its preachers was an enterprise logically deducible from the scientia Dei et beatorum, “the...