The problem with drawing lines in the sand is that once they're crossed, you have to respond to stay credible. So it is with President Barack Obama and any use of chemical weapons by the Syrian regime against its own people.
Yet, if the Iraq War taught us anything, it's that we need near-certainty that such weapons exist, and that any intervention has an achievable goal and is worth our sacrifice.
While the White House acknowledged for the first time Thursday that it had received evidence that Syrian President Bashar Assad may have deployed chemical munitions, there is no definitive proof yet. Experts caution that only a small amount of the nerve agent sarin was found, and there are no reports of widespread casualties.
Some hawks in Congress are jumping the gun by calling for aggressive action. It's much easier to talk about establishing no-fly zones, arming rebel groups and securing chemical weapons sites than actually do so.
There's no simple solution to assure Assad's downfall and stem the bloodshed in Syria's 2-year-old civil war, which has claimed an estimated 70,000 lives. One reason the United States has contributed only nonlethal aid to the rebels is that it's exceedingly difficult to make sure materiel goes only to those friendly to the West, not militants with ties to al-Qaida.
Obama repeated Friday that the use of chemical weapons would cross a line, but made clear that before any intervention, more evidence must be gathered and the international community must be consulted.
"This is going to be a long-term proposition," he said.
After our costly wars in Iraq and Afghanistan and the rise of fundamentalists after Hosni Mubarak's overthrow in Egypt, we ought to be far more circumspect about our ability to produce the change we want and to predict the consequences of U.S. involvement.
The president is right to be extremely cautious about intervening again in that part of the world.