Revolutionary socialism is the socialist doctrine that social revolution is necessary in order to bring about structural changes to society. More specifically, it is the view that revolution is a necessary precondition for a transition from capitalism to socialism. Revolution is not necessarily defined as a violent insurrection; it is defined as seizure of political power by mass movements of the working class so that the state is directly controlled by the working class as opposed to the capitalist class and its interests. Revolutionary socialists believe such a state of affairs is a precondition for establishing socialism and orthodox Marxists believe that it is inevitable but not predetermined.
Revolutionary socialism encompasses multiple political and social movements that may define "revolution" differently from one another. These include movements based on orthodox Marxist theory, such as De Leonism, impossibilism, and Luxemburgism; as well as movements based on Leninism and the theory of vanguardist-led revolution, such as Maoism, Marxism-Leninism, and Trotskyism. Revolutionary socialism also includes non-Marxist movements, such as those found in anarchism, revolutionary syndicalism, and democratic socialism.
It is used in contrast to the reformism of social democracy, which is not anti-capitalist in form. Revolutionary socialism is opposed to social movements that seek to gradually ameliorate the economic and social problems of capitalism through political reform. Revolutionary socialism also exists in contrast to the concept of small revolutionary groups seizing power without first achieving mass support, termed Blanquism.
The proletariat, the lowest stratum of our present society, cannot stir, cannot raise itself up, without the whole superincumbent strata of official society being sprung into the air. Though not in substance, yet in form, the struggle of the proletariat with the bourgeoisie is at first a national struggle. The proletariat of each country must, of course, first of all settle matters with its own bourgeoisie. In depicting the most general phases of the development of the proletariat, we traced the more or less veiled civil war, raging within existing society, up to the point where that war breaks out into open revolution, and where the violent overthrow of the bourgeoisie lays the foundation for the sway of the proletariat. (...) The Communists fight for the attainment of the immediate aims, for the enforcement of the momentary interests of the working class; (...) The Communists disdain to conceal their views and aims. They openly declare that their ends can be attained only by the forcible overthrow of all existing social conditions. Let the ruling classes tremble at a Communistic revolution.-- Karl Marx, Friedrich Engels, The Communist Manifesto, 1848
Twenty-four years after The Communist Manifesto, Marx and Engels admitted that in developed countries "labour may attain its goal by peaceful means". Marxist scholar Adam Schaff argued that Marx, Engels and Lenin have expressed such view "on many occasions". By contrast, the Blanquist view emphasised the overthrow by force of the ruling elite in government by an active minority of revolutionaries, who then proceed to implement socialist change, disregarding the state of readiness of society as a whole and the mass of the population in particular for revolutionary change.
In 1875, the Social Democratic Party of Germany (SPD) published a somewhat reformist Gotha Program, which was attacked by Marx in Critique of the Gotha Program, where he reiterated the need for dictatorship of the proletariat. The reformist viewpoint was introduced into Marxist thought by Eduard Bernstein, one of the leaders of the SPD. From 1896 to 1898, Bernstein published a series of articles entitled "Probleme des Sozialismus" ("Problems of Socialism"). These articles led to a debate on revisionism in the SPD and can be seen as the origins of a reformist trend within Marxism.
In 1900, Rosa Luxemburg wrote Social Reform or Revolution, a polemic against Bernstein's position. The work of reforms, Luxemburg argued, could only be carried on "in the framework of the social form created by the last revolution". In order to advance society to socialism from the capitalist 'social form', a social revolution will be necessary:
Bernstein, thundering against the conquest of political power as a theory of Blanquist violence, has the misfortune of labeling as a Blanquist error that which has always been the pivot and the motive force of human history. From the first appearance of class societies, having class struggle as the essential content of their history, the conquest of political power has been the aim of all rising classes. Here is the starting point and end of every historic period...In modern times, we see it in the struggle of the bourgeoisie against feudalism.-- Rosa Luxemburg, Social Reform or Revolution
Vladimir Lenin attacked Bernstein's position in his What Is To Be Done? When Bernstein first put forward his ideas, the majority of the SPD rejected them. The 1899 Congress of the SPD reaffirmed the Erfurt programme as did the 1901 congress. The 1903 congress denounced "revisionist efforts".
On 4 August 1914, the SPD members of the Reichstag voted for the government's war budget while the French and Belgium socialists publicly supported and joined their governments. The Zimmerwald Conference in September 1915, attended by Lenin and Leon Trotsky, saw the beginning of the end of the uneasy coexistence of revolutionary socialists and reformist socialists in the parties of Second International. The conference adopted a proposal by Trotsky to avoid an immediate split with the Second International. At first opposed to it, in the end Lenin voted for Trotsky's resolution to avoid a split among anti-war socialists.
In December, 1915 and March, 1916, eighteen Social Democratic representatives, the Haase-Ledebour Group, voted against war credits and were expelled from the Social Democratic Party. Liebknecht wrote Revolutionary Socialism in Germany in 1916, arguing that this group was not a revolutionary socialist group despite their refusal to vote for war credits, further defining in his view what was meant by a revolutionary socialist.
Many revolutionary socialists argue that the Russian Revolution of October 1917 led by Lenin and Trotsky follows the revolutionary socialist model of a revolutionary movement of the immense majority. By contrast, the October revolution is  portrayed as a putsch or coup d'état along the lines of Blanquism.
Revolutionary socialists, particularly Trotskyists, argue that the Bolsheviks only seized power as the expression of the mass of workers and peasants, whose desires are brought into being by an organised force--the revolutionary party. Marxists such as Trotskyists argue that Lenin did not advocate seizing of power until he felt that the majority of the population, represented in the soviets, demanded revolutionary change and no longer supported the reformist government of Alexander Kerensky established in the earlier revolution of February 1917:
Lenin, after the experience of the reconnoiter, withdrew the slogan of the immediate overthrow of the Provisional Government. But he did not withdraw it for any set period of time, for so many weeks or months, but strictly in dependence upon how quickly the revolt of the masses against the conciliationists would grow.-- Leon Trotsky, Lessons of October, Chapter Four: "The April Conference"
For these Marxists, the fact that the Bolsheviks won a majority (in alliance with the Left Socialist-Revolutionaries) in the second all-Russian congress of Soviets--democratically elected bodies--which convened at the time of the October revolution, shows that they had popular support of the masses of workers, peasants and soldiers, the vast majority of Russian society.
In his pamphlet Lessons of October, first published in 1923, Trotsky argued that military power lay in the hands of the Bolsheviks before the October Revolution was carried out, but this power was not used against the government until the Bolsheviks gained mass support.
The mass of the soldiers began to be led by the Bolshevik party after the July days of 1917 and followed only the orders of the Military Revolutionary Committee under the leadership of Trotsky in October (also termed the Revolutionary Military Committee in Lenin's collected works). Yet Trotsky only mobilised the Military Revolutionary Committee to seize power on the advent of the Second All-Russian Congress of Soviets of Workers' and Soldiers' Deputies, which began on 25 October 1917.
Following the October Revolution, the Communist International (also known as the Third International) was founded. This International became widely identified with communism, but also defined itself in terms of revolutionary socialism. However, in 1938 Trotskyists formed the Fourth International because they thought that the Third International was lost to Stalinism--this latter International became identified with revolutionary socialism.
Emerging from the Communist International, but critical of the post-1924 Soviet Union, the Trotskyist tradition in Western Europe and elsewhere uses the term "revolutionary socialism". For instance, in 1932 the first issue of the first Canadian Trotskyist newspaper, The Vanguard, published an editorial intitled "Revolutionary Socialism vs Reformism". Today, many Trotskyist groups advocate "revolutionary socialism" as opposed to reformism and are considered--and consider themselves--"revolutionary socialists". Luxemburgism is another revolutionary socialist tradition.