The White Terror in Hungary was a two-year period (1919–1921) of repressive violence by counter-revolutionary soldiers, carried out to crush any opposition supportive of Hungary’s short-lived Communist republic. Many of the victims of the White Terror were Jewish. During the White Terror, tens of thousands were imprisoned without trial and as many as 5,000 people were killed.
At the end of World War I, the political configuration of the Hungarian state was forced into swift and radical change. The Austro-Hungarian Empire, of which Hungary had been a powerful member, collapsed. The victorious Entente powers took steps to carve out Hungary’s ethnically mixed border regions and grant them to the Kingdom of Serbs, Croats and Slovenes, Czechoslovakia, and Romania – efforts which resulted in Hungary’s losing two thirds of its land area, and one third of its Hungarian-speaking nationals. These losses together with the postwar socioeconomic upheaval catalysed deep feelings of humiliation and resentment among many Hungarians.
In this volatile atmosphere, the nation’s fledgling efforts to form a single stable government failed. In March 1919, a government of communists, taking over from a Social Democrat-Communist coalition, established the Hungarian Soviet Republic. The Communist Party of Hungary, led by Béla Kun, had the most influence in the republic, although the government was ostensibly led by the Social Democratic-Communist coalition. Kun’s government lasted less than four months, eventually ending upon the Romanian invasion. During this period, heightened political tension and suppression led to arrests and executions in what came to be known as the Red Terror. This led to a lowering in support of the government. Hungary attempted to retain Slovakia and Transylvania, but Romanian troops invaded Hungary, eventually reaching Budapest in August 1919. Upon the invasion, most Hungarian communists, including Kun, went in exile.
First phase (1919)
In the south of the country, an alternative government formed to replace the Hungarian Soviet Republic. Leading the armed wing of this new government, the " National Army ", was Miklós Horthy, one-time Admiral of the Austro-Hungarian Navy.
Among the officers who answered Horthy’s call were ultra-nationalist soldiers who mounted a campaign of atrocities in a retaliation to the Red Terror ; to eliminate communist supporters and frighten the population into obedience to the new order.
The pogroms and mass murders were carried out by units of the "National Army" commanded by Mihály Horthy; paramilitary organisations also committed killings, especially the "Hungarian Awakening". 
These units, commonly known as the "White Guard," carried out a campaign of murder, torture and humiliations. Summary executions of people they suspected of communist allegiance were common; these victims were often hanged in public places to serve as a warning to others. But the White Guard’s definition of who was an enemy of the state was a broad one. They also preyed upon peasants, upon the politically liberal, and very often upon Jews, who were broadly blamed for the revolution because much of the communist leadership had been Jewish.
The most notorious of unit commanders was Pál Prónay, who by all accounts . Others included Gyula Ostenberg, Anton Lehár, and Ivan Hejjas, who focused his efforts on the Hungarian plain around the town of Kecskémet. Their detachments were part of the National Army, but tended to function as personal battalions following a fanatical loyalty to their commanders.
Hardest hit were the regions of Transdanubia, the wider area of Horthy's headquarters in Siófok, and in the lowlands between the Danube and the Theiss rivers, where mass murders which aroused international attention were committed in Kecskémet and Orgovány. 
Second phase (post-1919)
The National Army invaded Budapest in November 1919, and four months later Horthy became Regent of the newly established Kingdom of Hungary. But, far from discontinuing their campaigns, the reactionary units expanded and continued terrorising their targets for almost two more years; politically motivated violence devolved into grudge-murders and kidnappings for profit. White Guard officers began to vie for power among themselves, and plotted for one another’s assassinations. Horthy’s biographer, Thomas L. Sakmyster, concluded that Horthy looked the other way in 1919 while the White Guard officers raged through the countryside.
End of the White Terror
By 1920 the terror receded noticeably.  In 1921, Pál Prónay was prosecuted for crimes related to the White Terror. After Prónay joined a failed attempt to restore the Habsburg king, Charles I of Austria to Hungary’s throne, his battalion was disbanded.
Despite the disbandment of the Prónay battalion, in subsequent years sporadic attacks occurred.