Based on what U.S. officials knew 20 years ago, they were absolutely justified in taking military action to evict Saddam Hussein from power in Iraq. Twenty years later, the world remains, on rough balance, better off because they did. Despite all the United States and its allies got wrong operationally in Iraq in the past two decades, it also did a lot of things right and for good reasons — and with some markedly good results. WHAT IS THE MOST IMPORTANT LESSON OF THE IRAQ WAR? On March 19, 2003, there were several big reasons why decision-makers were right to order an assault on Saddam’s forces. And there were at least eight good results that flowed from the eviction of Saddam and the long-running allied presence in Iraq. Alas, not all those positive results lasted,...