Sixteen years ago this week, the George W. Bush administration sent U.S. forces crashing into Iraq without a real plan for what to do after defeating Saddam Hussein. It is easy to forget that moment’s fervent convictions about the Iraqi dictator’s supposed weapons of mass destruction and links to al Qaeda—and the calamity into which Iraq soon descended. It all seems to belong to another era. The debate today is more about how we got out of Iraq than how we got in.

The idea that the U.S. would do anything like Iraq again feels...