Socialism in one country was a Soviet state policy to strengthen socialism within the country rather than socialism globally. Given the defeats of the 1917–1923 European communist revolutions, Joseph Stalin developed and encouraged the theory of the possibility of constructing socialism in the Soviet Union alone. The theory was eventually adopted as Soviet state policy.

As a political theory, its exponents argue that it contradicts neither world revolution nor world communism. The theory opposes Leon Trotsky's theory of permanent revolution and the communist left's theory of world revolution.

Initially, all leading Soviet figures including Stalin agreed that the success of world socialism was a precondition for the survival of the Soviet Union. Stalin expressed this view in his pamphlet, "Foundations of Leninism." However, he would later change this position in December 1924 during the succession struggle against Trotsky and the Left Opposition.

The theory was criticized by Trotsky and Grigory Zinoviev as antithetical to Marxist principles while the theoretical framework was supported by Nikolai Bukharin.

The defeat of several proletarian revolutions in countries like Germany and Hungary ended Bolsheviks' hopes for an imminent world revolution and prompted them to focus on developing socialism in the Soviet Union alone, as advocated by Joseph Stalin. In the first edition of The Foundations of Leninism (1924), Stalin was still a follower of the classical Marxist idea that revolution in one country is insufficient. Vladimir Lenin died in January 1924 and by the end of that year in the second edition of the book Stalin's position started to turn around as he claimed that "the proletariat can and must build the socialist society in one country".

In April 1925, Nikolai Bukharin elaborated the issue in his brochure Can We Build Socialism in One Country in the Absence of the Victory of the West-European Proletariat? and the Soviet Union adopted socialism in one country as state policy after Stalin's January 1926 article On the Issues of Leninism. 1925–1926 signaled a shift in the immediate activity of the Communist International from world revolution towards a defense of the Soviet state. This period was known up to 1928 as the Second Period, mirroring the shift in the Soviet Union from war communism to the New Economic Policy.

In his 1915 article On the Slogan for a United States of Europe, Lenin maintained that proletarian victory would be uneven and arrive through individual capitalist nations' conversions to socialism. Lenin further elaborated that the working class must implement socialism nationally where they can without waiting on a coordinated international unison of revolutions.

Grigory Zinoviev and Leon Trotsky vigorously criticized the theory of socialism in one country. In particular, Trotskyists often claimed and still claim that socialism in one country opposes both the basic tenets of Marxism and Lenin's particular beliefs that the final success of socialism in one country depends upon the revolution's degree of success in proletarian revolutions in the more advanced countries of Western Europe. Notably, they cited Lenin's words at the March 1918 Seventh Congress where Lenin had argued that the success of the Soviet revolution inextricably depended on the success of socialist revolutions in other countries. However, in this address, Lenin had emphasized being realistic and pragmatic rather, cautioning against wishful thinking that would place too much faith in the immediate success of a world revolution.