Thin Blue Line
https://en.wikipedia.org/wiki/Thin_blue_line
The "thin blue line" is a term for the police that is used to assert that they are the line which keeps society from descending into violent chaos.
The term began as an allusion to the British infantry regiment The Thin Red Line during the Crimean War in 1854. The regiment of Scottish Highlanders — wearing red uniforms — held off a Russian cavalry charge. The "blue" in "thin blue line" refers to the blue color of the uniforms of many police departments.