During World War II, the Kingdom of Hungary engaged in the military occupation, then annexation, of the Bačka, Baranja, Međimurje and Prekmurje regions of the Kingdom of Yugoslavia. These territories had all been under Hungarian rule prior to 1920, and had been transferred to Yugoslavia as part of the post–World War I Treaty of Trianon. They now form part of several states: Yugoslav Bačka is now part of Vojvodina, an autonomous province of Serbia, Yugoslav Baranja and Međimurje are part of modern-day Croatia, and Yugoslav Prekmurje is part of modern-day Slovenia. The occupation began on 11 April 1941 when 80,000 Hungarian troops crossed the Yugoslav border in support of the German-led Axis invasion of Yugoslavia that had commenced five days earlier. There was some resistance to the Hungarian forces from Serb Chetnik irregulars, but the defences of the Royal Yugoslav Army had collapsed by this time. The Hungarian forces were indirectly aided by the local Volksdeutsche, the German minority, which had formed a militia and disarmed around 90,000 Yugoslav troops. Despite only sporadic resistance, Hungarian troops killed many civilians during these initial operations, including some Volksdeutsche. The government of the newly formed Axis puppet state, the Independent State of Croatia, subsequently consented to the Hungarian annexation of the Međimurje area, which dismayed the Croat population of the region.

The occupation authorities immediately classified the population of Bačka and Baranja into those that had lived in those regions when they had last been under Hungarian rule in 1920 and the mostly Serb settlers who had arrived since the areas had been part of Yugoslavia. They then began herding thousands of local Serbs into concentration camps and expelled them to the Independent State of Croatia, Italian-occupied Montenegro, and the German-occupied territory of Serbia. Ultimately, tens of thousands of Serbs were deported from the occupied territories. This was followed by the implementation of a policy of "magyarisation" of the political, social and economic life of the occupied territories, which included the re-settlement of Hungarians and Székelys from other parts of Hungary. "Magyarisation" did not impact the Volksdeutsche, who received special status under Hungarian rule, and in Prekmurje the Hungarian authorities were more permissive towards ethnic Slovenes.

Small-scale armed resistance to the Hungarian occupation commenced in the latter half of 1941 and was answered with harsh measures, including summary executions, expulsions and internment. The insurgency was mainly concentrated in the ethnic-Serb area of southern Bačka in the Šajkaška region, where Hungarian forces avenged their losses. In August 1941 a civilian administration took over the government of the "Recovered Southern Territories" (), and they were formally annexed to Hungary in December. In January 1942 the Hungarian military conducted raids during which they killed over 3,300 people, mostly Serbs and Jews.

In March 1944, when Hungary realised that it was on the losing side in the war and began to negotiate with the Allies, Germany took control of the country, including the annexed territories, during Operation Margarethe I. This was followed by the collection and transport of the remaining Jews in the occupied territories to extermination camps, resulting in the deaths of 85 per cent of the Jews in the occupied territories. Prior to their withdrawal from the Balkans in the face of the advance of the Soviet Red Army, the Germans evacuated 60,000–70,000 Volksdeutsche from Bačka and Baranja to Austria. Bačka and Baranja were restored to Yugoslav control when the Germans were pushed out of the region by the Red Army in late 1944. Međimurje and Prekmurje remained occupied until the last weeks of the war.

Background

At the Paris Peace Conference following the conclusion of World War I, the Entente Powers signed the Treaty of Trianon with Hungary after the breakup of Austria-Hungary. Among other things, the treaty defined the border between Hungary and the newly created Kingdom of Serbs, Croats and Slovenes (KSCS, renamed the Kingdom of Yugoslavia in 1929). It divided the previously Hungarian-ruled regions of Banat, Bačka and Baranja between Hungary, the KSCS, and Romania, and transferred the Međimurje region and about two thirds of the Prekmurje region from Hungary to the KSCS. Sizable numbers of Hungarians and Volksdeutsche remained in the areas incorporated into the KSCS. Between 1918 and 1924, 44,903 Hungarians (including 8,511 government employees) were deported to Hungary from the territories transferred to Yugoslavia, and approximately 10,000 Yugoslav military settlers (, lit. Salonika volunteers), mainly Serbs, were settled in Bačka and Baranja by the Yugoslav government. During the interwar period Hungary agitated for a revision of the borders agreed in the Treaty of Trianon, and relations between the two countries were difficult. On 22 August 1938, Czechoslovakia, Romania and Yugoslavia agreed to a revision of Trianon that allowed Hungary to re-arm itself.

Demographics Prior to the occupation, the most recent Yugoslav census had been taken in 1931. This census used language as the primary criterion, and counted all speakers of Serbo-Croatian as one group, rather than recognising distinct Serb, Croat, Bosnian Muslim, Macedonian and Montenegrin nationalities. Aligning data on religious affiliation with the linguistic data has been used by scholars to determine approximate numbers of Serbs and Croats in the 1931 census, by counting those of the Roman Catholic denomination as Croats.

According to the 1931 census, the territories of Bačka and Baranja had a combined population of 837,742. This included between 275,014 and 283,114 Hungarians, and between 185,458 and 194,908 Volksdeutsche. Hungarians therefore made up around one-third of the population of these territories, with the Volksdeutsche comprising slightly less than one-quarter. According to the historian Dr. Krisztián Ungváry, the 1931 census showed that the population of Bačka and Baranja included 150,301 Serbs and 3,099 Croats. This corresponds to a Serb population of about 18 per cent. These figures vary considerably from the combined Serb and Croat population of 305,917 provided by Professor Jozo Tomasevich, corresponding to 36.5 per cent of the population. The 1931 census figures for Međimurje and Prekmurje show a total population of 193,640, of which 101,467 (52.2 per cent) were Croats, 75,064 (38.7 per cent) were Slovenes, and 15,308 (8 per cent) were Hungarians.

Developments 1938–1941 Between 1938 and 1940, following German–Italian mediation in the First and Second Vienna Awards, and the Hungarian invasion of Carpatho-Ukraine, Hungary enlarged its territory. It absorbed parts of southern Czechoslovakia, Carpathian Ruthenia and the northern part of Transylvania, which the Kingdom of Romania ceded. One of the ethno-cultural areas that changed hands between Romania and Hungary at this time was the Székely Land. The support that Hungary received from Germany for these border revisions meant that the relationship between the two countries became even closer. On 20 November 1940, Hungary formally joined the Axis Tripartite Pact. On 12 December 1940, at the initiative of the Prime Minister, Count Pál Teleki, Hungary concluded a friendship and non-aggression treaty with Yugoslavia. Although the concept had received support from both Germany and Italy, the actual signing of the treaty did not, as Germany's planned invasion of Greece would be simplified if Yugoslavia could be neutralised. After the Yugoslav military coup of 27 March 1941, when the Germans asked the Hungarian Regent, Miklós Horthy, for clearance to launch one of their armoured thrusts using Hungarian territory, Teleki was unable to dissuade the Regent. Concluding that Hungary had disgraced itself irrevocably by siding with the Germans against the Yugoslavs, Teleki shot and killed himself. Horthy informed Hitler that evening that Hungary would abide by the friendship treaty with Yugoslavia, though it would likely cease to apply should Croatia secede and Yugoslavia cease to exist.

Invasion

On 10 April 1941, the Independent State of Croatia (, NDH) was established in Zagreb by the Ustaše. That day Horthy and the new Prime Minister of Hungary László Bárdossy issued a joint declaration that Yugoslavia had ceased to exist, releasing Hungary from its obligations under the non-aggression pact and the Treaty of Trianon. According to the declaration Hungarian troops would act to "protect the Hungarians who live in the south parts from the anarchy" of the April War which had begun there several days earlier when Italian and German troops invaded. The following day the Hungarian 3rd Army began occupying those regions of Yugoslavia using the Mobile, IV and V Corps, with I and VII Corps in reserve. That day (11 April), the headquarters of the 3rd Army informed that of the German 2nd Army that Hungarian forces had crossed the frontier north of Osijek and near Subotica.

The German army's rapid manoeuvres during the invasion had forced the tactical withdrawal of Yugoslav forces facing Hungarian army units and there was no significant fighting between the two armies. The Hungarian forces advanced south to the Danube between Vukovar and the confluence with the Tisza without any real military resistance. Serb Chetnik irregulars fought isolated engagements, and the Hungarian General Staff considered irregular resistance forces to be their only significant opposition.

On 12 April, the Hungarian 1st Parachute Battalion captured canal bridges at Vrbas and Srbobran. Meanwhile, Sombor was captured against determined Chetnik resistance, and Subotica was also captured. This, the first airborne operation in Hungarian history, was not without incident. The battalion's aircraft consisted of five Italian-made Savoia-Marchetti SM.75 transport aircraft formerly with the civilian airline MALERT, but pressed into service with the Royal Hungarian Air Force (, MKHL) at the start of the European war. Shortly after takeoff from the airport at Veszprém-Jutas on the afternoon of 12 April, the command plane, code E-101, crashed with the loss of 20 or 23 lives, including 19 paratroopers. This was the heaviest single loss suffered by the Hungarians during the Yugoslav campaign.

On 13 April, the 1st and 2nd Motorised Brigades occupied Novi Sad, then pushed south across the Danube into the northern part of Croatian Syrmia capturing Vinkovci and Vukovar on 18 April. These brigades then drove southeast to capture the western Serbian town of Valjevo a day later. Other Hungarian forces occupied the Yugoslavian regions of Prekmurje and Međimurje. A later American assessment concluded that German forces had to take the brunt of the fighting, observing that Hungarian forces had "displayed great reluctance to attack until the enemy had been soundly beaten and thoroughly disorganized by the Germans." When a Yugoslav delegation signed an armistice with German and Italian representatives at Belgrade on 17 April, the Hungarians were represented by a liaison officer, but he did not sign the document because Hungary was "not at war with Yugoslavia." The armistice came into effect at noon the next day. News of the success of the Hungarian armed forces in Yugoslavia was welcomed in the Hungarian Parliament. German forces occupied a narrow slice of northeastern Prekmurje along the German–Yugoslav border, which included four Volksdeutsche villages. In mid-June 1941, this area was absorbed into Reichsgau Steiermark.