Kate Woodsome, “Decoding Netanyahu’s ‘Red Line’ Against Iran” (Voice of America, Sep. 28, 2012)
One of the expression’s earliest appearances came in the 1850s, when the “thin red line” was used to describe the British army at the battle of Balaclava in the Crimean War, according to Ben Zimmer, a language columnist for The Boston Globe newspaper.
“There was a regiment of Scottish soldiers who wore red coats, and they were holding off the Russians in the battle,” he said. “They became known as the ‘thin red line,’ and that became a famous expression to refer to the British army.” …
“Kav adom,” the Hebrew equivalent of “red line,” might have been the first appearance of the phrase in the region, said Zimmer.
“The earliest example that I’ve seen is from 1975, and a quote from the Israeli foreign minister, Yigal Allon, who said at the time, ‘Washington has managed to draw a red line, which all the Arab countries know they must not cross, then America is not going to sacrifice Israel for Arab support,'” Zimmer said, adding that Netanyahu may be using “red line” because of its historical resonance.