An interesting piece from the OED site on tracking the first use of the phrase for the entry on 'red line'.
"The phrase thin red line, referring literally or metaphorically to the British army - from the former colour of their uniforms - is fairly familiar in the English language; so when we wanted to add it to the OED under the entry red line, I didn't imagine that it would be too hard to track down to its first recorded occurrence. But it proved to be a slippery customer."
"The phrase thin red line, referring literally or metaphorically to the British army - from the former colour of their uniforms - is fairly familiar in the English language; so when we wanted to add it to the OED under the entry red line, I didn't imagine that it would be too hard to track down to its first recorded occurrence. But it proved to be a slippery customer."
