Eventually Netanyahu backed down and declared a new “red line.”
Yet Israel has not attacked despite Iran stepping over the red line of starting enrichment in Fordow.
Panetta said the U.S. red line was that Iran would not be allowed to cross this Rubicon and go on to develop a nuclear weapon.
also red-line, "mark in red ink," 1820, from red (adj.1) + line (v.). Specific sense of "deny loans to certain neighborhoods based on ethnicity" is from 1973, on notion of lines drawn on maps. Used earlier in reference to insurance company practices (1961) and in World War II military slang in reference to a red line drawn through a soldier's name for some infraction, thus denying his pay. Related: Redlined; redlining.