Not long after the 2003 U.S. invasion of Iraq, a European friend asked me: Why is the United States behaving like a revisionist power when it so clearly benefits from the status quo? The only answer that I could offer was that, rightly or wrongly, American leaders no longer viewed the status quo as beneficial following the 9/11 attacks. The Sept. 11 attacks crystallized a certain then-contemporary anxiety that had not previously translated into policy—the idea that threats to the United States’