Investigating the Origins of the Phrase "Red Line"

Sometime in the next week or so the world will finally learn one way or the other what it means to cross a red line.

http://news.nationalgeographic.com/news/2013/09/130904-red-line-syria-obama-chemical-weapons-sarin-gas/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+ng%2FNews%2FNews_Main+%28National+Geographic+News+-+Main%29