At this point, it is safe to say that — short of definitive evidence of large-scale regime-directed chemical weapons use, or threats to Turkey, a U.S. treaty ally — it is highly unlikely that the United States will intervene militarily in Syria’s civil war. There are many reasons for this, including an American populace exhausted with nearly a dozen years of continuous warfare, senior military officials deeply opposed to an open-ended mission while still fighting in Afghanistan and confronting the threat of Islamic militants regrouping in southwest Libya, and a president who adheres to former Defense Secretary Robert Gates’s semi-serious dictum: “Every administration gets one preemptive war against a Muslim country.”

However, the most significant explanation of America’s unwillingness to attack Syria is that the level of military force that officials and policymakers are willing to employ would not materially change the outcome of the civil war. The threshold of force that would have to be used — as well as the sheer numbers of advanced, lethal weapons that would have to be supplied to the armed opposition — to assure the toppling of Assad, will not be forthcoming. The course and outcome of Syria’s civil war is simply not that important of a national interest for the United States to take the lead and catalyze a military coalition or weapons-supplying role.