One of the stock responses to complicated questions for many a modern Europeanist is to answer "the French Revolution." This can be an answer that is too pat, but the Revolution was a major sea change in European history and cast a long shadow over the politics in the following centuries. The color red's association with various left-wing causes is a case in point.

Red flags had several connotations in the eighteenth century such as "prepare for battle," but in ancien regime France, a red flag meant the declaration of martial law. The French National Guard flew the red flag when an anti-royalist demonstration got too large and unruly in Paris in July 1791. The resulting clashes resulted in over two score of dead. The National Assembly had authorized the declaration of marital law, but the more radical Jacobins used both the declaration and the red flag as a rallying point for pushing the revolution further. The Jacobin Club repurposed the red flag as a symbol of the blood shed by the martyrs but also as a warning sign not of martial law, but of continued revolution. Although the Tricolour was the official emblem of France during the Revolution, red became the symbol connected to the Jacobins. Not only did the red flag connect with the July 1791 events, but also with older European symbols such as the red Phyrgian cap - seen in this preserved example- which associated red with liberty.

Red soon became the common symbol for both revolution and insurgency in the post-1815 years. The intertwined symbolism of both a call to action and the blood of martyrs was irresistible. A good many of the revolutionaries in 1848 across Europe adapted the red flag as a sign of the people rising up against a tyrannical order. Karl Marx would write of the crushed French 1832 revolution, "Only after being dipped in the blood of the June insurgents did the tricolor become the flag of European revolution- the red flag!" Other 1848ers would take up Marx's call and push for the red flag as a sign of revolution. The red flag had become a stock trope for revolution by the 1850s. Melville's novella Billy Budd describes how the Nore mutineers used the red flag to transmute "the flag of founded law and freedom defined, into the enemy's red meteor of unbridled and unbounded revolt." Victor Hugo's novel Les Miserables also features mentions of the red flag among the 1832 revolt along with other symbols of 1789 like the Phyrgian cap. Carlye's popular history of the Revolution likewise propagated the connection of the red flag with extreme revolutionary ideas and the overthrow of the existing order.

Given such cultural and historical connotations, it was no surprise that the Communards of 1871 flew the red flag over their section of Paris. The Communards saw themselves as acting within the larger historical tradition of the French Revolution and the Jacobins. This is seen in some of the preserved broadsheets produced by the Communards, such as this one which evokes Delacroix's painting Liberty Leading the People. The Commune's suppression likewise further entrenched the red flag as the color for revolution as they became martyrs for many of the emerging socialist and Marxist parties throughout Europe. This preserved banner of the German SPD is typical of the red flags in this period and also notable for its connection between current politics and past suppression of revolution by promising "revenge for the officially suppressed and persecuted." The red flag was not only a strong political symbol with a clear revolutionary pedigree, it was also a universal one. Many socialist parties in the latter quarter of the nineteenth century saw themselves as part of a pan-European movement that stood for the common underclass of the repressed and downtrodden. The IWW's 1909 The Little Red Songbook has an explicitly international intent even if its songs delved into issues that were much more relevant for a North American context than a European one. The red flag stood for these universalist precepts as well as the cumulative historical legacies of Europe's revolutions.

Among these socialist groups at the turn of the century was a relatively small Russian one, the Russian Social Democratic Labor Party. The Bolshevik faction within the RSDLP saw itself as a vanguard for a revolution within Russia. The story of how the Bolsheviks managed to seize power in 1917 is a long and complicated one (and not really germane for this question!), but the red flag was one of the symbols they claimed as their own. The Bolsheviks likened themselves as the heirs to Europe's revolutionary traditions, drawing an imaginary line between the Jacobins, the 1848ers, the Communards, and all the way to the October Revolution. This 1921 poster makes the connection between present revolutionaries and the past martyrs clear with the caption "the martyrs of the Paris Commune were resurrected under the red banner of the Soviets!" The red flag became the natural emblem Lenin and company gravitated to as a symbol for their movement and for global revolution. From there, the red flag soon became the dominant symbol and motif for most communist parties.

The symbolism of the red flag is of course both striking and obvious; red = blood. But it also represented a revolutionary patrimony for many communist and left-wing groups in the twentieth century. Many political parties on the left wanted to connect up to their historical predecessors. The construction of this revolutionary genealogy was invariably selective in its treatment of past revolutions. Those revolutionaries who cited the Jacobins as their political ancestors seldom explored the Terror or explained away the Jacobins' political violence as necessary given the circumstances. The beauty of the red flag for left groups was not only its manifest symbolism, but in its constant use by various revolutions throughout Europe's long nineteenth century that began in 1789.

u/kieslowskifan wrote an answer about this here : www.reddit.com/r/AskHistorians/comments/799uie/why_was_the_color_red_so_associated_with_communism