Is a red line still a red line?

One of the most embarrassing moments of the extremely embarrassing Barack Obama presidency came in the context of the Syrian civil war. In August 2012, Obama vowed that “a red line for us,” which would thereby necessitate some sort of American intervention, “is we start seeing a whole bunch of chemical weapons moving around or being utilized.” The 44th president continued: “That would change my calculus.”

Except it didn’t.

A year later, former Iran- and Russia-backed Syrian strongman...