Over time, Christendom became a philosophical and historical concept, not an operational principle of strategy or international order. That process was facilitated because the Christian world had originated a distinction between “the things which are Caesar’s” and “the things that are God’s,” permitting an eventual evolution toward pluralistic, secular-based foreign policies within a state-based international system, as we have seen in the previous two chapters. It was also driven by contingent circumstances, among them the relative unattractiveness of some of the modern crusading concepts
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