Let’s say that Obama agrees that NATO could be the key force of an air campaign in Syria—and that enough NATO members agree to go along. (In Kosovo, every member of the alliance, except Greece, played some kind of role.) What would be the war’s objectives? …

A few things are clear from Obama’s record as commander-in-chief: He tends to resist the use of military force. When he sees it as unavoidable, he tends to steer clear of grandiose objectives, and he demands that allied nations come along, even take the lead, especially if their interests in the conflict outweigh ours.

If Obama does use force in Syria, he will do so because of clear evidence that Assad’s regime has killed lots of civilians with chemical weapons. Two considerations will likely drive his decision, if it comes to that. First, he has drawn a “red line” on this issue, publicly, at least five times in the last year, and failure to follow through—especially after the latest revelations—would send confusing signals, at best, about U.S. resolve and credibility. Second, failure to respond would erode, perhaps obliterate, the taboo that the international community has placed on chemical weapons (especially nerve gas) since the end of World War I. I suspect that this factor may be more pertinent to Obama, who takes the issue of international norms very seriously.