In politics, the term "Left" derives from the French Revolution, as the anti-monarchist Montagnard and Jacobin deputies from the Third Estate generally sat to the left of the presiding member's chair in parliament, a habit which began in the French Estates General of 1789. Throughout the 19th century in France, the main line dividing Left and Right was between supporters of the French Republic and those of the monarchy. The June Days Uprising during the Second Republic was an attempt by the Left to assert itself after the 1848 Revolution, but only a small portion of the population supported this.
In the mid-19th century, nationalism, socialism, democracy and anti-clericalism became features of the French Left. After Napoleon III's 1851 coup and the subsequent establishment of the Second Empire, Marxism began to rival radical republicanism and utopian socialism as a force within left-wing politics. The influential Communist Manifesto by Karl Marx and Friedrich Engels, published in 1848, asserted that all human history is the history of class struggle. They predicted that a proletarian revolution would eventually overthrow bourgeois capitalism and create a classless, stateless, post-monetary communist society. It was in this period that the word "wing" was appended to both Left and Right.
In the United States, many leftists, social liberals, progressives and trade unionists were influenced by the works of Thomas Paine, who introduced the concept of asset-based egalitarianism, which theorises that social equality is possible by a redistribution of resources.
The International Workingmen's Association (1864–1876), sometimes called the First International, brought together delegates from many different countries, with many different views about how to reach a classless and stateless society. Following a split between supporters of Marx and Mikhail Bakunin, anarchists formed the International Workers' Association. The Second International (1888–1916) became divided over the issue of World War I. Those who opposed the war, such as Vladimir Lenin and Rosa Luxemburg, saw themselves as further to the left.
In the United States after Reconstruction, the phrase "the Left" was used to describe those who supported trade unions, the civil rights movement and the anti-war movement. More recently in the United States, left-wing and right-wing have often been used as synonyms for Democratic and Republican, or as synonyms for liberalism and conservatism respectively.
Since the Right was populist, both in the Western and the Eastern Bloc anything viewed as avant-garde art was called leftist in all Europe, thus the identification of Picasso's Guernica as "leftist" in Europe and the condemnation of the Russian composer Shostakovich's opera (The Lady Macbeth of Mtsensk District) in Pravda as follows: "Here we have 'leftist' confusion instead of natural, human music".