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 Asked about the growing insurgency in Iraq back in July 2003, then-President George W. Bush responded with a remark that will very likely appear in his obituary: "Bring 'em on." For Bush, it was merely a regrettable quip. For Iraqis, there were very real consequences. Thousands of fighters, many of them radicalized by the Iraq invasion and occupation itself, traveled from countries in the region to "bring it on" in Iraq's cities and neighborhoods, markets and mosques. In addition to the costs in lives to Iraqis and American troops, the conflict enabled al-Qaeda planners to develop and refine a set of practices against the most skilled military in the world -- Iraq became a kind of terrorist boot camp.

As early as 2003, tactics and techniques developed in Iraq -- improvised...