The Russian Civil War (Russian: Гражданская война в России, romanized: Grazhdanskaya voyna v Rossii) was a multi-party civil war in the former Russian Empire sparked by the 1917 overthrowing of the Russian Provisional Government in the October Revolution, as many factions vied to determine Russia's political future. It resulted in the formation of the Russian Socialist Federative Soviet Republic and later the Soviet Union in most of its territory. Its finale marked the end of the Russian Revolution, which was one of the key events of the 20th century.

The Russian monarchy ended with the abdication of Tsar Nicholas II during the February Revolution, and Russia was in a state of political flux. A tense summer culminated in the October Revolution, where the Bolsheviks overthrew the provisional government of the new Russian Republic. Bolshevik seizure of power was not universally accepted, and the country descended into a conflict which became a full-scale civil war in May–June 1918. The two largest combatants were the Red Army, fighting for the establishment of a Bolshevik-led socialist state headed by Vladimir Lenin, and the forces known as the White movement (and its White Army), led mainly by the right-leaning officers of the Russian Empire, united around the figure of Alexander Kolchak. In addition, rival militant socialists, notably the Ukrainian anarchists of the Makhnovshchina and Left Socialist-Revolutionaries, were involved in conflict against the Bolsheviks. They, as well as non-ideological green armies, opposed the Bolsheviks, the Whites and the foreign interventionists. Thirteen foreign states intervened against the Red Army, notably the Allied intervention, whose primary goal was re-establishing the Eastern Front of World War I. Three foreign states of the Central Powers also intervened, rivaling the Allied intervention with the main goal of retaining the territory they had received in the Treaty of Brest-Litovsk with Soviet Russia.

The Bolsheviks initially consolidated control over most of the former empire. The Treaty of Brest-Litovsk was an emergency peace with the German Empire, who had captured vast swathes of the Russian territory during the chaos of the revolution. In May 1918, the Czechoslovak Legion in Russia revolted in Siberia. In reaction, the Allies began their North Russian and Siberian interventions. That, combined with the creation of the Provisional All-Russian Government, saw the reduction of Bolshevik-controlled territory to most of European Russia and parts of Central Asia. In 1919, the White Army launched several offensives from the east in March, the south in July, and west in October. The advances were later checked by the Eastern Front counteroffensive, the Southern Front counteroffensive, and the defeat of the Northwestern Army.

By 1919, the White armies were in retreat and by the start of 1920 were defeated on all three fronts. Although the Bolsheviks were victorious, the territorial extent of the Russian state had been reduced, for many non-Russian ethnic groups had used the disarray to push for national independence. In March 1921, during a related war against Poland, the Peace of Riga was signed, splitting disputed territories in Belarus and Ukraine between the Republic of Poland on one side and Soviet Russia and Soviet Ukraine on the other. Soviet Russia invaded all the newly independent nations of the former empire or supported the Bolshevik and socialist forces there, although the success of such invasions was limited. Estonia, Latvia, and Lithuania all repelled Soviet invasions, Ukraine and Belarus were divided (as a result of the Polish–Soviet War), while Armenia, Azerbaijan and Georgia were occupied by the Red Army. By 1921, the Bolsheviks had defeated the national movements in Ukraine and the Caucasus, although anti-Bolshevik uprisings in Central Asia lasted until the late 1920s.

The armies under Kolchak were eventually forced on a mass retreat eastward. Bolshevik forces advanced east, despite encountering resistance in Chita, Yakut and Mongolia. Soon the Red Army split the Don and Volunteer armies, forcing evacuations in Novorossiysk in March and Crimea in November 1920. After that, fighting was sporadic until the war ended with the capture of Vladivostok in October 1922, but anti-Bolshevik resistance continued with the Muslim Basmachi movement in Central Asia and Khabarovsk Krai until 1934. There were an estimated 7 to 12 million casualties during the war, mostly civilians.

The Russian Empire fought in World War I from 1914 alongside France and the United Kingdom (Triple Entente) against Germany, Austria-Hungary and the Ottoman Empire (Central Powers).

The February Revolution of 1917 resulted in the abdication of Emperor Nicholas II of Russia. As a result, the Russian Provisional Government formed by a coalition of centrist parties was established, and soviets, elected councils of workers, soldiers, and peasants, were organized throughout the country, leading to a situation of dual power. The Russian Republic was proclaimed in September of the same year.

The Provisional Government, led by Socialist Revolutionary Party politician Alexander Kerensky, was unable to solve the most pressing issues of the country, most importantly to end the war with the Central Powers. A failed military coup by General Lavr Kornilov in September 1917 led to a surge in support for the Bolsheviks, who took control of the soviets, which until then had been controlled by the Socialist Revolutionaries. Promising an end to the war and "all power to the Soviets", the Bolsheviks then ended dual power by overthrowing the Provisional Government in late October, on the eve of the Second All-Russian Congress of Soviets of Workers' and Soldiers' Deputies, in what would be the second Revolution of 1917. The initial stage of the October Revolution which involved the assault on Petrograd occurred largely without any human casualties. Despite the Bolsheviks' seizure of power, they lost to the Socialist Revolutionary Party in the 1917 Russian Constituent Assembly election, and the Constituent Assembly was dissolved by the Bolsheviks in retaliation. The Bolsheviks soon lost the support of other far-left allies, such as the Left Socialist-Revolutionaries, after their acceptance of the terms of the Treaty of Brest-Litovsk presented by the German Empire. Conversely, a number of prominent members of the Left Socialist Revolutionaries had assumed positions in Lenin's government and led commissariats in several areas. This included agriculture (Kolegaev), property (Karelin), justice (Steinberg), post offices and telegraphs (Proshian) and local government (Trutovsky). The Bolsheviks also reserved a number of vacant seats in the Soviets and Central Executive for the Menshevik and Left Socialist Revolutionaries parties in proportion to their vote share at the Congress. The dissolution of the Constituent Assembly was also approved by the Left Socialist Revolutionaries and anarchists, both groups were in favour of a more radical democracy.