Back in August of 2012, then President Obama caused something of a diplomatic sensation when he gave Syria a "‘red line’ warning on chemical weapons,” according to a Washington Post headline at the time. Speaking at an impromptu news conference, the President said that “a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus. That would change my equation.”

The Assad regime responded almost exactly one year later by striking the opposition-controlled area of Ghouta with rockets containing the chemical agent sarin killing hundreds of people, mainly civilians.