In Episode 1 of A Red Line in the Sand, a companion podcast to the book of the same time, author David Andelman sets the stage. He explains what red lines are and why they matter.

Red lines can be geographic, diplomatic, political, or - most dangerously - military. They can be concepts, trade pacts, or physical barriers. The Iron Curtain in Cold War Europe, the use of chemical weapons in Syria, the 38th parallel on the Korean peninsula, and the line between desert and jungle in Chad - all of these are red lines with potentially lethal consequences.