It is hard, if you've got a head and a heart, to come down against a strong U.S. response to Syria's use of chemical weapons against its civilian population. This is especially so if you believe that humanity stands at a door that leads only to darkness. Those who say, "But Saddam Hussein used chemical weapons—the taboo was broken long ago," are missing the point. When Saddam used gas against the Kurds it was not immediately known to all the world. It was not common knowledge. The world rued it in retrospect. Syria is different: It is the first obvious, undeniable, real-time, YouTubed use of chemical weapons....