There’s a lesson here. Humanitarian interventions seem to be limited and discrete. Threaten to massacre a city, and we block you. Use chemical weapons and we take them out. In practice, however, it doesn’t work that way. Once we enter a conflict on humanitarian grounds, anything short of well-executed regime-change tends to make us look weak.

By defining chemical weapons use as a “red line,” instead of one factor among many to be judged in context at the moment of use, we allow humanitarian concerns to compel huge, risky, and difficult-to-control adventures that go way beyond their initial stated purpose. The alternative is to come off looking ineffective by ignoring our declared limits of tolerance. In other words, all we achieve by drawing humanitarian lines before-the-fact is to surrender control of our own foreign policy.

Obama thought turning chemical weapons use into a red line in Syria would be cost free. Why would Assad be stupid enough to cross us? After all, hadn’t he seen what we did to Gaddafi? He had, but it didn’t matter. This is partly because Assad is desperately fighting for his life, and partly because Libya itself was a very mixed signal.