There are probably limits to Moscow’s deference to Mr. Assad’s blood lust, but it is unclear what they are. This is what makes an American escalation in Syria so dangerous.

American supporters of intervention, including the vice-presidential candidates, often say that the United States should create a no-fly zone in Syria to protect civilians from Mr. Assad and Russia’s bombs. But imagine how this might work: An American warplane enforcing a no-fly zone would risk fire from a Russian-made antiaircraft battery or fighter. (Just this week Russia shipped new antiaircraft systems to Syria.)

This risk clearly worries advocates for the use of force within the Obama administration. They are said to favor increased air support for the Syrian rebels that would avoid direct confrontation with the Russians. But small-scale, targeted bombing is unlikely to change Syrian behavior, so to be effective the strikes would have to escalate. (Alternatively, ineffective strikes could be ended, but this would make the United States look incompetent.) This would ultimately lead to a violent response, which would compel the United States to retaliate against Russian and Syrian government ground targets.

As conflict spiraled and casualties increased, so would international pressure for another costly, protracted and thankless American-led ground intervention to enforce peace, which domestic opinion in the United States would not support. While Russia’s real appetite for a political solution in the Syria conflict is unclear, it is wiser to test unknown political limits than unknown military ones.