President Obama clearly wishes to avoid intervening in Syria. His earlier stated red lines were written with lots of leeway for Assad: guns, tanks, helicopters, aircraft — all were cool. Only chemical weapons would force a U.S. response. Assad is now testing those red lines. Agent 15 is obviously a more dangerous substance than tear gas. Yet it’s probably not what intervention-wary American voters imagine when they hear the phrase “chemical weapon.”

The most-heard estimate of casualties from the Homs event is five. Saddam Hussein’s gas attack on the Kurdish town of Halabja in 1988 killed an estimated 5,000 people and left perhaps another 7,000 seriously ill.

If another Halabja is what President Obama had in mind to deter, then so far, he can say “mission accomplished.”

American policy is shaped most fundamentally by this dilemma: as awful a tyrant as Bashar Assad is, the most effective among the anti-Assad insurgents look even worse — Sunni chauvinist jihadis likely to be even more disturbing to peace and security in the region than the young Assad ever was. Acting against Assad seems most likely only to help them into power.

So it looks as if Assad has successfully carved more space for himself.