I haven’t written anything about Syria for a while, mostly for the simple reason that it’s just too depressing: a civil war that it seems the government can’t possibly win, but where it seems to have the capacity to hold out quite a bit longer – and do a lot more killing in the meantime.

But in the last week has come possibly the most disturbing news of all, as the regime of president Bashar al-Assad is alleged to have used chemical weapons against its opponents. Barack Obama had said as long ago as last August that bringing chemical weapons into play would constitute a “red line” that America was not willing to see crossed. Last Thursday, in a letter to Carl Levin, the chairman of the Senate armed services committee, the White House revealed that intelligence agencies “assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin.”

America, however, has not committed to doing anything about it. On the right, that’s seen as a sign of, at best, Obama’s weakness and incompetence, at worst of his fealty to his Marxist-Islamist overlords. But it seems to me there’s a much more obvious explanation, and one that the right desperately wants to avoid drawing attention to.

Ten years ago, American troops were still in their first flush of victory in Iraq and happily searching for the weapons of mass destruction that their leaders had assured them were there. Of course they weren’t, and we now know that those leaders never had any good reason to think that they were.

Obama, whose political success was closely bound up with his clear and prompt opposition to the invasion of Iraq, is determined that the same mistake not be made on his watch. Some of the passages from his press conference on Tuesday sound like direct references to the Iraq experience:

And when I am making decisions about America’s national security and the potential for taking additional action in response to chemical weapon use, I’ve got to make sure I’ve got the facts.  That’s what the American people would expect.

And if we end up rushing to judgment without hard, effective evidence, then we can find ourselves in a position where we can’t mobilize the international community to support what we do.  There may be objections even among some people in the region who are sympathetic with the opposition if we take action.  So it’s important for us to do this in a prudent way.

True enough. But there remains the question, even if Assad’s possession and use of weapons of mass destruction (which of course he denies) is proved beyond reasonable doubt, what can the US and other countries actually do about it?

The most sensible thing I’ve read on that subject is by Larry Derfner this week in +972 Magazine. Derfner writes from a left-wing Israeli perspective; he’s deeply worried about Syrian WMDs (especially biological weapons) but pessimistic about the chances of being able to intervene without making matters worse:

I can’t think of anything Israel, the United States or anybody else can do to ensure that Syria’s chemical and maybe biological weapons don’t come into the possession of Islamic terrorists. The prospective “no-fly zone” that a lot of Americans are talking about might make it harder for Assad to prosecute the war and thus bring down the level of killing – or it might not. At any rate, a no-fly zone is not going to remove the chemical/biological weapons from Syria or the jihadists who would like to have them.

His conclusion is that stopping the proliferation of WMDs has always depended mainly on deterrence, and that that will continue to be the case. The “least terrible option”, he suggests, is that Israel and the US should “refrain from attacking anything and anybody in Syria unless they attack first.”

The problem, of course, is that describing something as a “red line” and then doing nothing if someone crosses it is itself a failure of deterrence. Sometimes the logic of the situation requires doing something that in the short term is counter-productive, because weakening the deterrent for the future may have worse consequences still.

Probably the best that can be done is not to target WMDs directly, but to step up practical assistance to the Syrian opposition – and particularly to the most responsible elements within it – in the hope of getting the war over with as soon as possible. But that’s easier said than done.