Reports that Syria has indeed used such weapons has brought Obama's remark back into the news—and prompted Senator John McCain to say that the lack of U.S. response indicates that the "red line ... was apparently written in disappearing ink." McCain also applied the term to the alleged Israeli missile strikes into Syria: "Apparently the Syrians and Iranians have crossed a red line with the Israelis."

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 of English and journalism at the University of Delaware. One of the earliest recorded instances of anyone drawing a line in the sand took place in ancient Rome around 168 B.C.,