To put up a red flag is a metaphor for giving or sending a warning signal and has been used in this figurative way since the early 1700s. Since the 14th century, a red flag in naval warfare meant no quarter. If opposing ships did not surrender then no prisoners would be taken. Besieged castles and towns would also run up red flags to indicate that they would fight to the end. Red flags these days indicate danger or stop, as in modern motor racing etc. The Red Flag was also the universally accepted anthem and symbolism of Socialist and Communist movements from the mid-19th century onwards. See also Red.