Assessing the decision with the wisdom of hindsight, as historians will also do, likewise requires a decades-long period of waiting for critical facts to emerge. Reports that Saddam Hussein sent WMD to Syria before his downfall have yet to be verified or disproved. Only time will reveal whether Iraq has served as a magnet for terrorists who otherwise would have gone to the United States to perpetrate the next 9/11.
If Americans were asked on Sept. 12, 2001, what sacrifices they would make to prevent another massive terrorist attack, a large fraction no doubt would have been willing to accept the costs that have been incurred in Iraq. After all, 9/11 killed 3,000 people and caused economic damage that, according to some estimates, exceeded the costs of the Iraq war to date.
At the moment, we lack essential information about the war's impact on the international scene. The extent of foreign support and opposition, in 2003 and 2008, will not be known until the declassification of documents, since the true views of governments often differ sharply from their public postures. Libya and other countries may or may not have become more willing to cooperate with the US after watching Saddam Hussein fall.
In researching Lyndon Johnson's 1965 decision to fight in Vietnam, four decades after the fact, I discovered information that led to a fundamental reassessment of the decision. Contrary to what nearly all historians and journalists had written, every country in Asia aside from China's allies supported American intervention in Vietnam. Many of those countries made hitherto unknown offers of combat troops and military aid. US intervention in Vietnam persuaded the Indonesian military to oust the pro-Communist President Sukarno, one of America's greatest victories in the cold war.