Hungarian–Czechoslovak War

The Hungarian–Czechoslovak War or Northern Campaign (északi hadjárat) was fought between the Hungarian Soviet Republic and the First Czechoslovak Republic from April to June 1919.

Background
At the end of 1918, the final year of World War I, the collapse of Austria-Hungary led to the declaration of the formed Czechoslovakia. The newly established country wanted to ensure the success of their territorial demands and started an attack. During the war, the Hungarian Red Army fought separate battles against troops from Czechoslovakia and Romania, while France was also highly involved diplomatically in the conflicts, too. By its final stage, more than 120,000 troops on both sides were involved.

Appealing to Hungarians with promises of regaining the land lost until then to neighboring countries within a week of his rise to power, Béla Kun declared war upon Czechoslovakia, which would increase his domestic support by making good on his promise to restore Hungary's borders. The Hungarian army recruited men between 19 and 25 years of age. Industrial workers from Budapest volunteered. Many former Austro-Hungarian officers re-enlisted through patriotism. The Hungarian army moved its 1st and 5th artillery divisions (40 battalions) to Upper Hungary (partially modern day Slovakia). The Hungarian counterattack launched on 9 May 1919 in the area of Hatvan. On 20 May 1919, Colonel Aurél Stromfeld, attacked in force and routed Czechoslovak troops from Miskolc, also recapturing Košice and Prešov, thus successfully separated the Czechoslovak and Romanian armies from each other. Through this successful action, Hungary controlled territory up to its old northern borders; regained control of industrial areas around Miskolc, Salgótarján and Banská Štiavnica.

The proclamation of Slovak Soviet Republic and the moral collapse of the army
Despite communist promises on the restoration of the former borders of Hungary, the communist declared the establishment of the Slovak Soviet Republic in Prešov on 16 June 1919. After the proclamation of the Slovak Soviet Republic, the Hungarian nationalists and patriots soon realized that the new communist government had no intentions to recapture the lost territories, only to spread communist ideology and establish other communist states in Europe, and thus sacrificing Hungarian national interests. Despite the series of military victories against the Czechoslovak army, the Hungarian Red Army started to disintegrate due to this fundamental tension between patriots and communists during the establishment of the Independent Slovak Soviet Republic, and this concession shook the popular and military support of the communist government, particularly among professional military officers, patriots and nationalists in the Hungarian Red Army; even the chief of the general staff Aurél Stromfeld, in fact, resigned his post in protest.

Aftermath
In the face of advancing Hungarian troops, the Allies began to put pressure on the Hungarian government and, within three weeks with Kun's assurances of Russian support failing to materialize, Hungary was forced to withdraw from Slovakia after given an ultimatum from France together with a guarantee that Romanian forces would retreat from Tiszántúl. General Aurél Stromfeld resigned after the acceptance of Clemenceau's proposition of Hungary's new borders. Following a brief war between Hungary and Romania, Slovakia was incorporated into Czechoslovakia.