But Syria is not so congenial. To start with, the insurgents have attracted much less active support, and their sympathizers are scattered. "Here, there is not even a whole city, much less a medium-sized region, that we could work with to build a defensible area," Pape said. An outside force would have to capture a chunk of territory, which is a much harder -- and bloodier -- assignment than safeguarding an established zone.
Air power is generally unavailing in situations where government loyalists and rebels are cheek by jowl on the ground and devilishly hard to distinguish from cloud level. In that situation, ground forces are the way to go, but it would involve the likelihood of significant American casualties.
That prospect is a big deterrent, and it ought to be. One reason Obama got little pushback at home on Libya was that we didn't lose a single soldier. Syria would be different -- more like the invasion of Afghanistan. We might prevail, but at a much higher price than in Libya and only if we were willing to stay on indefinitely.
One reason the cost would escalate, said Pape, is that our invasion would look suspiciously like an act of conquest rather than altruism. After all, Syria has long been at odds with its neighbor, Israel, which happens to be our close ally.
We may regard the two countries as largely separate issues, but Syrians would suspect NATO forces of doing the dirty work of the hated Zionist entity. They would be encouraged in that notion by the mullahs in Tehran -- who would regard the Syria operation as a prelude to an attack on Iran and strive to help Assad.
Critics demand that Obama show "leadership" by doing something to help Syria's civilians. But sometimes leadership lies in knowing what not to do -- and then not doing it.