The October Revolution, officially known as the Great October Socialist Revolution under the Soviet Union, also known as the Bolshevik Revolution, was a revolution in Russia led by the Bolshevik Party of Vladimir Lenin that was a key moment in the larger Russian Revolution of 19171923. It was the second revolutionary change of government in Russia in 1917. It took place through an armed insurrection in Petrograd (now Saint Petersburg) on 7 November 1917 [O.S. 25 October]. It was the precipitating event of the Russian Civil War.

The October Revolution followed and capitalized on the February Revolution earlier that year, which had overthrown the Tsarist autocracy, resulting in a liberal provisional government. The provisional government had taken power after being proclaimed by Grand Duke Michael, Tsar Nicholas II's younger brother, who declined to take power after the Tsar stepped down. During this time, urban workers began to organize into councils (soviets) wherein revolutionaries criticized the provisional government and its actions. The provisional government remained unpopular, especially because it was continuing to fight in World War I, and had ruled with an iron fist throughout the summer (including killing hundreds of protesters in the July Days).

Events came to a head in the fall as the Directorate, led by the left-wing Socialist Revolutionary Party, controlled the government. The left-wing Bolsheviks were deeply unhappy with the government, and began spreading calls for a military uprising. On 10 October 1917 (O.S.; 23 October, N.S.), the Petrograd Soviet, led by Trotsky, voted to back a military uprising. On 24 October (O.S.; 6 November, N.S.) the government shut down numerous newspapers and closed the city of Petrograd in an attempt to forestall the revolution; minor armed skirmishes broke out. The next day a full scale uprising erupted as a fleet of Bolshevik sailors entered the harbor and tens of thousands of soldiers rose up in support of the Bolsheviks. Bolshevik Red Guards forces under the Military-Revolutionary Committee began the occupation of government buildings on 25 October (O.S.; 7 November, N.S.), 1917. The following day, the Winter Palace (the seat of the Provisional government located in Petrograd, then capital of Russia) was captured.

As the Revolution was not universally recognized, the country descended into the Russian Civil War, which would last until 1923 and ultimately lead to the creation of the Soviet Union in late 1922. The historiography of the event has varied. The victorious Soviet Union viewed it as a validation of their ideology, and the triumph of the worker over capitalism. During Soviet times, revolution day was a national holiday, marking its importance in the country's founding story. On the other hand, the Western Allies saw it as a totalitarian coup, which used the democratic Soviet councils only until they were no longer useful. The event inspired many cultural works, and ignited communist movements across Europe and globally. Many MarxistLeninist parties around the world still celebrate October Revolution Day.

The Socialist Federative Republic of Councils in Hungary (Hungarian: Magyarországi Szocialista Szövetséges Tanácsköztársaság), due to an early mistranslation, it became widely known as the Hungarian Soviet Republic in English language sources (Hungarian: Magyar Szovjet-köztársaság), literally the Republic of Councils in Hungary (Hungarian: Magyarországi Tanácsköztársaság) was a short lived Communist state from 21 March 1919 until 1 August 1919 (133 days), succeeding the First Hungarian Republic. The head of government was Sándor Garbai, but the influence of the foreign minister Béla Kun from the Hungarian Communist Party was much stronger. Unable to reach an agreement with the Triple Entente, which maintained an economic blockade in Hungary, tormented by neighboring countries for territorial disputes, and invested by profound internal social changes, the soviet republic failed in its objectives and was abolished a few months after its existence. The presentation of the Vix Note led to the fall of the liberal Count Mihály Károlyi's government, which was by then devoid of significant support, and the proclamation of the soviet republic by 12 March. Its main figure was the Communist Béla Kun, despite the fact that in the first days the majority of the new government was Socialist. The new system effectively concentrated power in the governing councils, which exercised it in the name of the working class.The new regime failed to reach an agreement with the Triple Entente that would lead to the lifting of the economic blockade, the improvement of the new borders or the recognition of the new government by the victorious powers of World War I. A small volunteer army was organized mostly from Budapest factory workers and attempts were made to recover the lost territories at the hands of neighboring countries, an objective that aroused widespread support from worker class people in some larger cities, not only those more favorable to the newborn regime. For their part, the neighboring countries used the struggle against communism, first against the capitalist government of liberal Count Mihály Károlyi and later against the soviet republic, as a justification for their expansionist ambitions. Initially, thanks to patriotic support from conservative officers, the republican forces advanced against the Czechoslovakians in Slovakia, after suffering a defeat in the east at the hands of the Romanian Army in late April, which led to a retreat on the banks of the Tisza. In mid-June, the birth of the Slovak Soviet Republic was proclaimed, which lasted two weeks until the Hungarian withdrawal at the request of the Triple Entente. On 20 July, the soviet republic launched a new attack on the Romanian posts. After a few days in advance, the Romanians managed to stop the offensive, to break through the front and reach Budapest, the Hungarian capital, a few days after the end of the soviet republic, which was abolished on 2 August.The Hungarian heads of government applied doctrinal measures in both foreign and domestic policy that made them lose the favor of the majority of the population. The attempt of the new executive to profoundly change the lifestyle and the system of values of the population proved to be a resounding failure; the effort to convert Hungary, which still had the aftermath of the Habsburg period, into a socialist society was unsuccessful due to a number of elements, namely lack of time, experienced administrative and organizational staff, as well as inexperience, both political and economic, in some of the maneuvering activities. The attempt to win the sympathies of the peasants met with general indifference, since encouraging agricultural production and supplying the cities at the same time was not a process that could be completed in a short period of time. After the withdrawal from Slovakia, the application of some measures aimed at regaining popular support was ordered, without great success; in particular, the ban on the sale of alcoholic beverages was repealed, the delivery of some plots to peasants was planned without land, and attempts were made to improve the monetary situation and food supply. Unable to apply them, the soviet republic had already lost the support of the majority of the population between June and July, which led, together with military defeats, to its inexorable ruin. The failure of internal reform was joined by that of foreign policy; the political and economic isolation of the Triple Entente, the military failure in the face of neighboring countries and the impossibility of joining forces with the Red Army units contributed to the collapse of the soviet republic. The Socialist–Communist government was succeeded by an exclusively Socialist one on 1 August; the Communists left Budapest and went abroad,