Obama said it last August: Any use of chemical weapons by Syrian authorities in their ongoing civil war would be to cross a "red line"—and (presumably) invite American action.
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., during a conflict that, curiously enough, involved Syria. A Roman envoy named Popillius was sent to tell King Antiochus IV to abort his attack on Alexandria. When Antiochus tried to play for time, Popillius drew a line in the sand around him and told him he had to decide what he was going to do before he crossed it. He acceded to the Roman demands.
But just how the venerable line in the sand came to be red is a little unclear. One possibility is that a pundit borrowed the idea of the warning line on a gauge beyond which it is unsafe to rev up the machinery. Another explanation comes from the Battle of Balaclava on October 24, 1854, when the hopelessly outnumbered Sutherland Highlanders, the 93rd Highland Regiment, were told by their commander Sir Colin Campbell: "There is no retreat from here, men. You must die where you stand." And so they stood firm, wearing their scarlet tunics and awaiting the charge. In his breathless account of the battle, London Times correspondent William Russell wrote that all that remained between the charging Russians and the British regiment's base of operations was "a thin red streak tipped with steel."
There are a couple of well-known early American references to the line in the sand. In Tom Sawyer (1876) Tom draws such a line with his big toe and dared another boy to cross it (spoiler alert: he did). Colonel William Travis drew a fateful line in the sand at the Alamo in 1836, asking volunteers to cross the line and join him in remaining to fight despite hopeless odds.
The courage and steadfastness of the "thin red line" at Balaclava in staying the course was celebrated by poets from Tennyson ("The Charge of the Light Brigade") to Kipling ("Tommy") and over time became a metaphor for steely resolve and a firm stance.
In a lighthearted vein the metaphor shows up in Carry On … Up the Khyber (1968), a British comedy set in the days of the British Raj, when Private Jimmy Widdle, fearing an attack, paints a red line on the ground and declares, "They'll never get past this."
In the modern-day political arena, Yagoda has found uses dating back to 1987, with references to "red lines" in conflicts between Chad and Libya, and in a 1999 article in the New York Timesthat reported that Muslim clerics in Iran had set out a "red line for the revolution" that no one should cross.
Related: Word in the News: Jihad