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. The whole world knew of it the morning after it happened, through horrified, first-person accounts, from videos of hospital workers and victims' families.

The world this time cannot "not know," or claim not to know. And though Bashar Assad has made his pro forma denials, it does not seem believable that this was not a government operation. The rebels may or may not be wicked enough to use such weapons, but it is hard to believe they are capable.

When something like this happens and the world knows and does not respond, you won't get less of it in the future, you'll get more. And the weapons will not only be chemical.

So the question: What to do?

After 10 days of debate in Europe and America, the wisest words on a path forward have come from the Pope. Francis wrote this week to Vladimir Putin, as the host of the G-20. He damned "the senseless massacre" unfolding in Syria and pleaded with the leaders gathered in St. Petersburg not to "remain indifferent"—remain—to the "dramatic situation." He asked the governments of the world "to do everything possible to assure humanitarian assistance" within and without Syria's borders.