Assisted by Iran, Hezbollah and Russia, Mr. Assad indiscriminately fired missiles into civilian neighborhoods. More than 100,000 people have been killed, with millions more injured or displaced from their homes. Mr. Assad does not seem to be losing.
The dangerous outcomes that Mr. Obama worried might be precipitated by U.S. involvement have mostly come about in the absence of such involvement. Syria has become a haven for thousands of fighters affiliated with al-Qaida. Violence has spread to neighboring states, especially Lebanon and Iraq. U.S. allies Turkey and, especially, Jordan are in danger of being overwhelmed by hundreds of thousands of Syrian refugees.
Now, according to Doctors Without Borders and other credible sources, weapons of mass destruction apparently have been used on a scale not seen since Saddam Hussein went after his Kurdish population in 1988, with Mr. Assad seemingly calculating he has little to fear from crossing Mr. Obama's "red line."
The U.S. president has been correct from the start that the Syria crisis offered no good options to U.S. policymakers. That has become only more true as the United States has remained aloof. With little support from the West, opposition forces that espouse a multisectarian, democratic Syria have found themselves challenged and weakened not only by Mr. Assad's forces but also by more radical Islamist fighters. This shouldn't have come as a surprise.
But the fact that Syria offers no perfect outcomes or options does not mean that all possible outcomes are equally undesirable. It remains as much in the United States' interest now as two years ago to see more moderate forces prevail. This can't be achieved with one or two volleys of cruise missiles. It will require patience and commitment.
The United States can't dictate the outcome in Syria, and it would be foolish to send ground troops in an effort to do so. But by combining military measures targeting forces using chemical weapons with training, weapons supplies and diplomacy, it could exercise considerable influence. Military action should be seen as one component of a policy that finally recognizes a U.S. interest in helping to shape Syria's future.