In politics, a red flag is predominantly a symbol of socialism, communism, Marxism, trade unions, left-wing politics, and anarchism. The originally empty or plain red flag has been associated with left-wing politics since the French Revolution (1789–1799).Brink, Jan ten [ Robespierre and the Red Terror], (1899).
Socialists adopted the symbol during the Revolutions of 1848 and it was first used as the flag of a new authority by the Paris Commune of 1871. The flag of former Soviet Union, introduced after the Russian Revolution, is explicitly inspired by the plain red flag. The red flag is also used as a symbol by some democratic socialists and social democrats, for example the League of Social Democrats of Hong Kong, the French Socialist Party and the Social Democratic Party of Germany. The Labour Party in Britain used it until the late 1980s. It was the inspiration for the socialist anthem, The Red Flag.
Prior to the French Revolution and in some contexts even today, red flags or banners were seen as a symbol of defiance and battle.
Red color as a combat or revolt symbol in Europe goes back to the turn of the millennia and before. In the Middle Ages, ships in combat flew a long red streamer called the baucans to signify a fight with no quarter.
The red cap was a symbol of popular revolt in France going back to the Jacquerie of 1358. The color red became associated with patriotism early in the French Revolution due to the popularity of the tricolour cockade, introduced in July 1789, and the Phrygian cap, introduced in May 1790. A red flag was raised over the Champ-de-Mars in Paris on July 17, 1791, by Gilbert du Motier, marquis de Lafayette, commander of the National Guard, as a symbol of martial law, warning rioters to disperse. As many as fifty anti-royalist protesters were killed in the fighting that followed.
Inverting the original symbolism, the Jacobins protested this action by flying a red flag to honor the "martyrs' blood" of those who had been killed.