Nir Rosen’s Aftermath, an extraordinary feat of reporting, follows the contagious spread of radicalism and sectarian violence that the U.S. invasion of Iraq and the ensuing civil war have unleashed in the Muslim world. Rosen—who the Weekly Standard once bitterly complained has “great access to the Baathists and jihadists who make up the Iraqi insurgency”— has spent nearly a decade among warriors and militants who have been challenging American power in the Muslim world. In Aftermath, he tells their story, showing the other side of the U.S. war on terror, traveling from the battle-scarred streets of Baghdad to the alleys, villages, refugee camps, mosques, and killing grounds of Jordan, Syria, Egypt, Lebanon, and finally Afghanistan, where Rosen has a terrifying encounter with...