It’s pretty predictable: When the U.S. leaves a power vacuum, other states rush to fill it. We just may not be happy with the results.

Case in point: Syria.

The Obama administration has not been willing to do much to help the rebels, pointedly ignoring their pleas for anti-aircraft missiles and no-fly zones to shut down Bashar Assad’s murderous air force. So enter the Saudis to provide “manpads” (i.e., portable anti-aircraft weapons) to the rebels.

Although the Saudis claim to be providing these weapons only to secular elements of the Free Syrian Army, it’s hard to have much confidence in their track record. Recall how they armed the most extreme jihadists in Afghanistan in the 1980s.

Even if the missiles in question go only to “moderate” rebel fighters, the danger that they will fall into the wrong hands goes up. And that could be a catastrophe in the making, given what an enticing target civil airliners, such as those that land at Ben Gurion Airport in Israel, are for terrorists armed with manpads.

That is why the Obama administration has been warning against supplying these portable and deadly missiles to the rebels. But if it had really wanted to stop the arms flow, the U.S. should have offered an alternative method of taking down Assad’s airpower–most likely in the form of a no-fly zone enforced by the U.S. and its allies. Failing that, the Saudis are taking matters into their own hands–yet another reason to be concerned about the consequences of American retreat.