The Hungarian Soviet Republic, also known as the Socialist Federative Soviet Republic of Hungary was a short-lived state that existed from 21 March 1919 to 1 August 1919 (133 days), succeeding the First Hungarian Republic. The Hungarian Soviet Republic was a small rump state run by communists and socialists which, at its time of establishment, controlled approximately only 23% of Hungary's historic territory. The head of government was Sándor Garbai, but the influence of Béla Kun of the Party of Communists in Hungary was much stronger. Unable to reach an agreement with the Triple Entente, which maintained an economic blockade of Hungary, in dispute with neighboring countries over territorial disputes, and beset by profound internal social changes, the Hungarian Soviet Republic failed in its objectives and was abolished a few months after its existence. Its main figure was the Communist Béla Kun, despite the fact that in the first days the majority of the new government consisted of radical Social Democrats. The new system effectively concentrated power in the governing councils, which exercised it in the name of the working class.

The new Communist government 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 territories lost to neighboring countries, an objective that had widespread support from many working-class people in some larger cities, not only those favorable to the new regime. Initially, thanks to patriotic support from conservative officers, the republican forces advanced against the Czechoslovaks in north Hungary, 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 a Hungarian withdrawal at the request of the Triple Entente. Later that month, there was an attempted coup by the Social Democrats of Budapest, which was retaliated by the communist government. On 20 July, the republic launched a new attack on the Romanian posts who were deep in Hungary at the Tisza river. After a few days of the Hungarian advance, the Romanians managed to stop the offensive and break through the Hungarian lines. Kun and most of the government fled to Vienna. The Social-Democrat–Communist government was succeeded by an exclusively Social Democratic one on 1 August. The communists left Budapest and went abroad. Despite the opposition from the Entente, the Romanians entered Budapest, the Hungarian capital, on 4 August.

The Hungarian heads of government applied controversial doctrinal measures in both foreign (internationalism instead of national interests during wartime) and domestic policy (planned economy and heightened class struggle) 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; After the withdrawal from Slovakia, the application of some measures aimed at regaining popular support was ordered, but without great success; in particular, the ban on the sale of alcoholic beverages was repealed, and attempts were made to improve the monetary situation and food supply. Unable to apply these policies effectively, the republic had already lost the support of the majority of the population between June and July, which led, together with the military defeats, to its downfall. The failure of internal reform was compounded by the political and economic isolation imposed on Hungary by the Triple Entente, the military failures against neighboring countries, and the impossibility of joining forces with the Red Army because of the ongoing Russian Civil War contributed to the collapse of the Soviet Republic.

The leaders of the Hungarian Soviet Republic adhered to the Marxist belief in global proletarian revolution, which they saw as imminent. Convinced that national borders, countries and nations themselves were bourgeois constructs that would soon disappear in favor of a unified socialist world order, they prioritized defending their communist regime, spreading revolutionary ideals, and attempting to export revolution through military campaigns. They prioritized class struggle and loyalty to international communism, openly dismissed Hungarian patriotism as bourgeois sentiment and an obstacle to world revolutionary unity. They rejected Hungarian territorial integrity, national pride and identity in their rhetoric, and framed the 'patriotism' and the existence of Hungarian national identity as a distraction from their ultimate goal: a borderless global proletarian state united under international communism.

One practical manifestation of this internationalist doctrine was the Northern Campaign. This military campaign, conducted in the territory of the former Upper Hungary, ostensibly "aimed" to reclaim lost lands. However, its true objective — the establishment of an independent Slovak Soviet Republic — remained concealed from the Hungarian public until the Slovak Soviet republic's official proclamation on June 16.

Austria-Hungary's Habsburg monarchy collapsed in 1918, and the independent First Hungarian Republic was formed after the Aster Revolution. The official proclamation of the republic was on 16 November and the liberal Count Mihály Károlyi became its president. Károlyi struggled to establish the government's authority and to control the country. The Hungarian Royal Honvéd army still had more than 1,400,000 soldiers when Mihály Károlyi was announced as prime minister of Hungary. Károlyi yielded to United States president Woodrow Wilson's demand for pacifism by ordering the unilateral self-disarmament of the Hungarian Army. This happened under the direction of Minister of War Béla Linder, on 2 November 1918. Due to the unilateral disarmament of its army, Hungary was to remain without a national defence at a time of particular vulnerability. The Hungarian self-disarmament made the occupation of Hungary directly possible for the relatively small armies of Romania, the Franco-Serbian army and the armed forces of the newly established Czechoslovakia.

On the request of the Austro-Hungarian government, an armistice was granted to Austria-Hungary on 3 November by the Allies. Military and political events changed rapidly and drastically thereafter. On 5 November, the Serbian Army, with the help of the French Army, crossed southern borders. On 8 November, the Czechoslovak Army crossed the northern borders. On 13 November, the Romanian Army crossed the eastern borders of the Kingdom of Hungary. During the rule of Károlyi's pacifist cabinet, Hungary lost control over approximately 75% of its former pre-World War I territories (325,411 km²) without armed resistance and was subject to foreign occupation. For their part, the neighboring countries used the so-called "struggle against communism", first against the capitalist and liberal government of Count Mihály Károlyi and later against the soviet republic of communist Bela Kun, as a justification for their expansionist ambitions.

An initial nucleus of a Party of Communists in Hungary had been organized in a hotel on 4 November 1918, when a group of former Hungarian prisoners of war and other communist proponents formed a Central Committee in the downtown of Moscow. Led by Béla Kun, the inner circle of the freshly established party returned to Budapest from Moscow on 16 November. On 24 November, they created the Party of Communists from Hungary (Hungarian: Kommunisták Magyarországi Pártja, KMP). The name was chosen instead of the Hungarian Communist Party because the vast majority of supporters were from the urban industrial working class in Hungary which at the time was largely made up of people from non-Hungarian ethnic backgrounds, with ethnic Hungarians a minority in the new party itself. The party recruited members while propagating its ideas, radicalising many members of the Social Democratic Party of Hungary in the process. By February 1919, the party numbered 30,000 to 40,000 members, including many unemployed ex-soldiers, young intellectuals and ethnic minorities.