Republican presidential candidate and former New York City Mayor Rudy Giuliani pauses while speaking at the Institute of Politics at Saint Anselm College in Manchester, New Hampshire, November 5, 2007.
Brian Snyder / Reuters

We are all members of the 9/11 generation.

The defining challenges of the twentieth century ended with the fall of the Berlin Wall. Full recognition of the first great challenge of the twenty-first century came with the attacks of September 11, 2001, even though Islamist terrorists had begun their assault on world order decades before. Confronted with an act of war on American soil, our old assumptions about conflict between nation-states fell away. Civilization itself, and the international system, had come under attack by a ruthless and radical Islamist enemy.

America and its allies have made progress since that terrible day. We have

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