The 2003 Iraq War lasted less than three weeks. It began in the early morning hours of March 20, when American missiles struck Baghdad. By April 9, U.S. forces had advanced into Baghdad. By April 15 Iraqi leader Saddam Hussein had vanished, and U.S. and allied officials pronounced the end of major combat operations. President George W. Bush repeated that pronouncement in a May 1 speech.

Although the war itself was short, arguments over whether it was justified had been made for months and years prior to the March 20 attacks—and continued long after the war was over. The roots of the crisis that led to the war go back at least as far as 1990, when George H.W. Bush was president and Iraq invaded and occupied its neighbor Kuwait. In the 1991 Persian Gulf War, Iraqi forces were driven from Kuwait by a...