Now the Syrian Army appears to have done precisely what he warned them not to do. According to the U.S. government's reckoning, some 1,429 people – including 426 children – were killed by deadly sarin gas during a rocket attack by the Syrian Army on August 21: a pretty specific crossing of the red line. As a result, red lines and the likely consequences of crossing them have been much in the news. Just this week, at a news conference in Stockholm, Obama declared: "I didn't set a red line. The world set a red line."

The phrase "red line" appears to be an adaptation of a much older metaphor—a "line drawn in the sand," according to Ben Yagoda, a professor