The war left two political vacuums. One is in Iraq, where the post-Saddam government can barely hold its own against ISIS and other sectarian forces. The other is in the United States. The war discredited Republican management of the presidency. George W. Bush devoted six years to a massive policing and nation-building project, and he failed. His failure destroyed the GOP’s self-confidence and led many Americans to turn away, embracing alternatives on the left and right.