The end of U.S. combat operations in Iraq this month has been an anti-climax. There is no defining image for us to remember – nothing like the last helicopter out of Saigon or the last Russian tank out of Afghanistan. 50,000 American military trainers remain in Iraq.

Even if there has been little drama, historians may conclude that this was an important moment. As the U.S. is pulling back from Iraq, strategists and policy-makers...