'Margarethe I' was the German occupation of Hungary to prevent the country’s defection for the Axis alliance (9/12 March 1944).

Adolf Hitler felt himself forced to make this move by the increasingly ambivalent position of the Hungarian administration of prime minister Dr Miklós Kállay de Nagykálló, which was playing both sides of the field in an attempt to mitigate for Hungary the ever worsening position of the Axis.

Vezérfökapitány Miklós Horthy de Nagybánya, the regent of Hungary, was closeted with the German leader at Kressheim at the time 'Margarethe I' was launched by 11 German divisions, and was thus faced with a fait accompli when he swore in a new administration headed by Vezérezredes Döme Sztójay, the pro-Nazi former ambassador to Berlin.

During 1943, as the Allies gained the grand strategic and strategic initiatives in their war with Germany, the latter’s allies (notably Italy, Hungary, Romania and Bulgaria) and the co-belligerent Finland became increasingly concerned that a successful outcome to their commitment to the Axis cause was at best problematical and at worst impossible. These nations thereupon began to seek ways of disentangling themselves from their association with Germany. The first visible result of this process was he Armistice of Cassibile of 8 September 1943 by which Italy ended her war with the Allies.

The Hungarian administration of prime minister Kállay had by this time started to sound out the possibility of a similar arrangement, but had been unable to prevent the German intelligence services from learning of this. Thus Germany entertained strong suspicions that Hungary, and probably also Romania, would follow the Italian lead, possibly after an Allied amphibious assault on the Balkans.

Edmund Veesenmayer, who have been secretly placed in Hungary during 1943 by Joachim Ribbentrop, the German foreign minister, to keep watch on political developments in that country, several times reported that the primary problem was Hungarian Jewry which, he claimed on 10 December, was responsible for Hungarian defeatism. Veesenmeyer urged that the current Hungarian government should be replaced by an administration, still headed by Horthy, the regent, but more compliant with the wishes and needs of Germany. Veesenmeyer believed that the right tool for the task of subordinating Hungary entirely to Germany was Béla Imrédy, who had been prime minister in 1938/39 and was still a major figure on the extreme right wing of Hungarian politics.

By September 1943 the operations staff of the Oberkommando der Wehrmacht had planned the 'Margarethe I' occupation of Hungarian and detailed the forces required. The Oberkommando der Wehrmacht had also created a similar 'Margarethe II' plan for the simultaneous occupation of Romania, whose oilfields were vital to Germany’s continued ability to sustain her war effort. However, the situation on the Eastern Front at this time now meant that the simultaneous launches of 'Margarethe I' and 'Margarethe II' would be impossible for lack of the forces required.

After they had digested the implications of the inter-Allied 'Eureka' conference in Tehran, the Germans decided that an Allied invasion of the Balkans was no longer imminent, and their relations with Hungary and Romania eased. In February 1944, however, Adolf Hitler ordered the implementation of 'Margarethe I' in the first half of March under the control of the Oberbefehlshaber 'Südost', Generalfeldmarschall Maximilian Reichsfreiherr von Weichs. It was planned that, depending on the specific situation, Romanian and Slovak troops would also be involved, but this element was later removed from the plan.

The German plans were thrown into disorder by the huge success of the offensives which the Soviets launched fin the spring of 1944. The Oberkommando der Wehrmacht had planned to conceal the real nature of 'Margarethe I' for as long as possible under the guise of a 'redeployment' of German forces onto Hungarian soil, and then to seize Hungary’s most important economic and industrial centres. The changing situation persuaded Hitler that he should meet Horthy, and this was arranged for 18 March at Klessheim, near Salzburg in Austria. Here Hitler demanded a change of government and the right to station German troops in Hungary. Horthy refused to sign the agreement, but was persuaded in a private meeting with Hitler that resistance would be futile.

In the early hours of 19 March, as Hungarian delegation was returning to Budapest, German troops began their invasion. The formations committed to 'Margarethe I', under the immediate control of General Hermann Foertsch, who had just ceased to be Weichs’s chief-of-staff, were General Hubert Lanz’s XXII Gebirgskorps advancing from Serbia and Slavonia, General Hans Dehner’s LXIX Corps from Croatia, General Walter Krüger’s LVIII Reserve-Panzerkorps from the Vienna area and Generalmajor Oswin Grolig’s LXXVIII Corps from the Kraków area. The Germans achieved total surprise and attained their objectives without meeting any resistance.

The occupying forces ordered the immediate disestablishment of many Hungarian command elements, and the disarmament of several Hungarian formations. Vezérezredes Ferenc Szombathelyi, the Hungarian army chief-of-staff, who had been with Horthy at Klessheim, had radioed orders that the Germans were to be welcomed, and Horthy’s special train was deliberately stopped on its way to Budapest so that the Hungarian regent could be greeted by a German guard of honour. But Horthy refused to step down as regent, and from this time onward worked increasingly against the interests of Germany.

Germany’s new senior political figure in Hungary was Dietrich von Jagow, the commander of the German troops in Hungary was General Hans von Greiffenberg, and SS General Otto Winkelmann became the Höherer SS- und Polizeiführer (higher SS and police leader) in Hungary. For the planned deportation of Hungarian Jews the Sondereinsatzkommando 'Eichmann' was established, and between April and July 1944 this sent nearly 440,000 Hungarian Jews to concentration and extermination camps.

On 22 March, after long negotiations, Horthy appointed as new prime minister Vezérezredes Döme Sztójay. In the meantime some 3,000 people deemed hostile by the German security authorities had been arrested under the instigation of SS-Obergruppenführer und General der Polizei und Waffen-SS Ernst Kaltenbrunner, the head of the Reichssicherheitshauptamt (Reich Main Security Office).