While there are a number of plausible labels that might be attached to the 20th century, in terms of social history it was clearly the age of the working class. For the first time, working people who lacked property became a major and sustained political force. This rupture was heralded by Pope Leo XIII -- leader of the world's oldest and largest social organization -- in his encyclical Rerum Novarum in 1891. The Pope noted that the progress of industry had led to `the accumulation of affluence among the few and misery (inopia) among the multitude'; but the period had also been characterized by the `greater self-confidence and tighter cohesion' of the workers.  On a global level, trade unions gained a foothold in most big industrial enterprises, and in many other firms too. Working-class parties became major electoral forces -- sometimes dominant ones -- in Europe and its Australasian offshoots. The October Revolution in Russia provided a model of political organization and social change for China and Vietnam. Nehru's India set itself the avowed goal of following a `socialist pattern of development', as did the majority of post-colonial states. Many African countries spoke of building `working-class parties' when they could boast no more proletarians than would fill a few classrooms.
May Day began on the streets of Chicago in 1886, and was celebrated in Havana and other Latin American cities as early as 1890. Organized labour proved to be an important force in the Americas, even if it was usually kept subordinate. The US New Deal marked a confluence between enlightened liberalism and the industrial working class, which succeeded in organizing itself during the Depression years through heroic struggles. Samuel Gompers may have epitomized the parochial craft unionism which preceded the New Deal, but he was a formidable negotiator on behalf of the skilled workers that his movement represented, and was honoured with a monument in Washington that exceeded any bestowed upon a workers' leader in Paris, London or Berlin. 
Mexico's small working class was not a leading actor in its Revolution -- though not a negligible one, either -- but the post-revolutionary elite expended much energy absorbing organized labour into its machinery of power. The Revolution's first president, Venustiano Carranza, forged his social base through a pact with the anarcho-syndicalist workers of Mexico City (the Casa del Obrero Mundial), and in the 1930s Lázaro Cárdenas gave the structures of the new order an explicitly workerist orientation.  While that could hardly be said of Getúlio Vargas and his `New State' in Brazil, a raft of progressive labour laws became one of its legacies. In Argentina, it was working-class mobilization, notably directed by Trotskyist militants, that brought Juan Perón to power, guaranteeing Argentine trade unionism -- or at least its leadership -- a major voice in the Peronist movement ever since. Bolivia's miners played a central role in the Revolution of 1952, and when tin production collapsed in the 1980s, the organizing skills of those obliged to seek work elsewhere provided Evo Morales and his coca growers with a spine of disciplined cadres.
Perhaps the greatest tribute to the centrality of the working class in the last century was paid by the most fanatical enemies of independent workers' movements, the Fascists. The idea of `corporatism' was vital to Mussolini's Italy: purporting to bring labour and capital together, in reality corralling labour into a field fenced off by capital and the state. Hitler's movement called itself the National Socialist German Workers' Party, and his Germany became the second country in the world -- trailing after the Soviet Union but ahead of Sweden -- to establish May Day as a public holiday, the `Day of German Labour'. In the first eighty years of the 20th century, workers could not be written off or dismissed. If you were not with them, you had to keep them under tight control.
Workers became heroes or models, not only for the artists of the left-wing avant-garde, from Brecht to Picasso, but also for more conservative figures, such as the Belgian sculptor Constantin Meunier -- creator of several statues depicting workers of different occupations, and of an ambitious `Monument to Labour', erected posthumously in Brussels in the presence of the King. In Germany, the Prussian officer-writer Ernst Jünger penned an admiring essay, `The Worker', in 1932, predicting the end of the Herrschaft (domination) of the third estate and its replacement by `the Herrschaft of the worker, of liberal democracy by labour or state democracy'. 
While the working-class century no doubt ended in defeat, disillusion and disenchantment, it also left behind enduring achievements. Democracy as a universal political model, violations of which nowadays require special pleading, is one. The Social Democratic labour movement was the main proponent of democratic reform, following the example of its Chartist predecessor. Before 1918, most liberals and all conservatives were convinced that democracy was incompatible with the preservation of private property, and thus demanded severe restrictions on the right to vote and the freedom of parliaments.  The defeat of Fascism by an inter-continental Popular Front of Communists, Liberals, Social Democrats and Conservatives such as Churchill and de Gaulle; the more protracted downfall of counter-revolutionary military dictatorships; and the demise of institutional racism in South Africa and the United States established the validity of global human rights. The right of wage-workers to organize and bargain collectively was another major gain of the post-war conjuncture. Conservative forces have chipped away at those advances recently in the US and the UK, but in the meantime their purchase has spread across the world, to the formal economic sectors in Africa and Asia; it remains strong in Latin America and in most of Europe.
The 20th century can never be understood without a full comprehension of its great revolutions, the Russian and the Chinese, with their profound repercussions for Eastern Europe, the Caribbean and much of East and Central Asia -- not to mention their influence on labour movements and social policy in Western Europe. Their assessment remains both politically controversial and, from a scholarly perspective, premature. Undoubtedly, these Revolutions gave rise to brutal repression and to episodes of arrogant modernist cruelty that resulted in vast suffering, such as the famines which took place during the rule of Stalin and Mao. Their geo-political achievements are equally beyond dispute -- though this is hardly a left-wing criterion of performance. Decaying, backward Russia, beaten by the Japanese in 1905 and the Germans in 1917, became the USSR: a state which defeated Hitler and established itself as the world's second superpower, appearing for a time to be a serious challenger to US primacy. The Chinese Revolution ended 150 years of decline and humiliation for the `Middle Kingdom', turning China into a global political force before its progress along the capitalist road made it the world's second-largest economy.
These 20th-century revolutions have left the world with at least four important progressive legacies. Firstly, their challenge had a crucial impact on post-war reform within the capitalist world: redistribution of land in Japan, Taiwan and South Korea; the development of social rights in Western Europe; and the `Alliance for Progress' reforms in Latin America -- all were inspired by the Communist threat. Secondly, the existence of a rival power bloc with its own ideology did much to weaken Euro-American racism and colonialism. Eisenhower would not have sent federal troops to enforce desegregation in Arkansas if he had not been concerned about winning the propaganda battle with Moscow. Two decades later, Cuban troops held back the South African army as it tried to conquer Angola, and the apartheid regime could not have been isolated so effectively without the shadow cast by the Soviet Union in global politics.
Thirdly, whatever may be said about the ruthless authoritarianism of its leaders, the Communist movement produced an extraordinary number of self-sacrificing, dedicated militants in every corner of the world. Their adulation of Stalin or Mao was wrong-headed, but very often they were the best -- sometimes the only -- friends of the poor and the downtrodden. This everyday commitment demands the respect of all progressives. Finally, and of more questionable significance, there is an organizational legacy which remains a factor in the modern world. The states of the two great revolutions may no longer be beacons of hope, but they are essential if some degree of geo-political pluralism is to be conserved (this includes post-Communist Russia). The persistence of Communist-led states after 1989 - 91 means that a socialist option remains open to some degree. If the rulers of the People's Republic were to conclude that China requires a socialist economic base to underpin its national strength, or that further progress along the capitalist road would imperil social cohesion, they still have the power and the resources to change track.
Communist parties or their descendants retain a foothold in many countries. Communism has a significant presence on the Indian political scene, albeit one that is splintered between competing forces: the Maoists pursue a guerrilla war in tribal regions, while the Communist Party of India (Marxist) is reeling from historic election defeats after its experience of state government in Kerala and West Bengal. There are substantial parties in Greece, Portugal, Japan, Chile and the Czech Republic. Greek and Portuguese Communists have played an important role in working-class mobilizations against the Eurozone's economic thumbscrew, and the SYRIZA coalition, led by former Euro-Communists, came a clos