Anyone who says we should "support the rebels" is making a wish, not a plan. Support them how? The one sure means of achieving regime change an all-out, Iraq-style invasion is out of the question. We could give heavy weapons, capable of shooting down Assad's planes and destroying his tanks, to some of the moderate rebel groups. But this materiel could end up in the hands of Islamist, anti-Western factions that seem a good bet to prevail in a post-Assad Syria.
What about imposing a no-fly zone? Some people talk about this option as if it were a breeze, but in fact it would be a major undertaking. Syrian air defenses, which are substantial, would have to be destroyed. The zone would have to be patrolled by U.S. or allied aircraft. And if Assad held on, there would be pressure for deeper American involvement.
A Reuters/Ipsos poll released Sunday found that about 60 percent of Americans believe the United States should stay out of Syria's civil war, while only 9 percent favor intervention. If it is proved that Assad used chemical weapons, the poll found, support for U.S. intervention rises to 25 percent. But 46 percent of those surveyed a large plurality said that even in the face of such proof, the United States should not act.
Given recent history, it should be no surprise that Americans are gun-shy. Obama came into office promising to end our long, costly wars in Iraq and Afghanistan. There is good reason to fear that Syria is the most slippery of slopes and to believe that the only sure way to avoid sliding into the middle of this brutal, messy war is to stay far away.
Despite all this, I unhappily conclude that Obama has to act.
The president was right to make chemical weapons use the "red line" that Assad must not cross. Upholding the principle that such weapons must never, ever be employed is so important that Obama, in my view, really has no choice.
Are the relatively few deaths caused by nerve gas really so different from the many more deaths caused by bullets, rockets and bombs? Yes, I believe they are.
There is an international consensus that chemical weapons, because of their potential for mass annihilation, are beyond the pale; any government that uses them will lose all legitimacy. If one tinhorn despot is allowed to get away with gassing his opponents, other thuggish strongmen a category of which there is no shortage will be emboldened to follow suit.
This is a case in which somebody has to be the world's policeman. Given Russia's alliance with Assad's regime and China's long-standing policy of indifference, the United Nations is almost sure to do nothing. France and Britain may step forward, as happened in Libya; but the essential military firepower and coordination will again be provided by the United States.
It makes me nervous when "Western intelligence sources" make assertions about weapons of mass destruction in faraway lands; see Iraq. U.N. investigators are probably reaching the site too late for a definitive determination, given that the evidence is as evanescent as the wind. But the pictures and the eyewitness testimony are clear, and Assad had a motive: Earlier this month, rebels launched a rocket attack on his convoy.
It will be difficult to design a missile strike that hurts Assad without drawing the United States into the war. But that is the thin line Obama must now walk.
Eugene Robinson's email address is email@example.com.