From The Second Great War: A Picture History, Houghton Mifflin, 1954
Up until 1941, the Kingdom of Hungary's policy toward the war had been one of uneasy neutrality. An authoritarian regency dominated by extreme nationalist elements, Hungary's government and populace were far more sympathetic to the Germans than to the Russians, but a fear of open warfare with either power had up to this point discouraged the country's monarchical leader, Regent Miklos Horthy, from formally picking a side. This hesitation was reinforced in mid-1940 after two of Hungary's neighbors, Czechoslovakia and Romania, fell respectively to the Russians and Germans. Horthy's wariness of an alliance with Germany did not fade until the Fall of France, after which Germany appeared unassailable.
Now assured of a German victory, Horthy gladly reached out to German Foreign Minister Konstantin von Neurath. Clandestine negotiations had nearly concluded for an official declaration of war against the Anti-Fascist Coalition when the Russians launched the Dniepr-Wisla Offensive, driving the Germans back hundreds of miles in a matter of three months; within days of the initial attack, Hungary withdrew from the negotiations, and by the time the Offensive concluded, eastern Poland was solidly in Russian hands, and Horthy sought to erase all evidence that he had ever seriously considered an alliance with Nazi Germany. Therefore, when Russian Foreign Commissar Alexandra Kollontai delivered an ultimatum—submit to an occupation by the Red Army, or face a full-scale invasion—the Regent reluctantly accepted, and ordered his military to yield to Russian forces.
Miklos Horthy, Regent of the Kingdom of Hungary.
While part of Hungary's military obeyed the edict out of loyalty to the Regent, most of the Army promptly ignored Horthy's edict. When Russian troops under Colonel-General Georgy Zhukov came pouring through the Romanian border on June 27, they resisted fiercely, mounting a surprising defeat of Russian forces outside the city of Szeged on July 6 and yielding Brettenbach to the invaders the same week after inflicting heavy casualties.
Alas, the brave resistance was not to last. Enraged that Horthy would betray Germany in such a way (and overlooking both the fact that no formal alliance with Hungary had been finalized, and that the Wehrmacht, still reeling from its crushing defeat in Poland, was in no position to invade yet another country), Hitler ordered an invasion of Hungary from eastern Austria, Operation Albatross, that commenced on July 8. This was a dismal failure: Austria, surrounded by heretofore unthreatening territories, had not been heavily militarized, and the already-weakened Germany had just finished transferring most of its free units from France to the Vistula, leaving few with which to conduct an invasion. German planners seemed to assume that the Hungarian military would welcome the invaders as allies against the Communist threat; however, they evidently failed to realize that to a significant portion of the Hungarian Army, a German invasion was barely, if at all, preferable to a Russian one.
General Geza Lakatos, leader of the contingent of the army that had remained loyal to Horthy, managed to attract more than a quarter of the rebellious units to defect from the anti-Horthy faction and travel to the western border to engage with the encroaching Germans, led by General Wilhelm Willemer. Though they took early losses, including the cities of Sopron on July 11 and Gyor three days later, their situation improved by the end of the month, when the Germans reached the country's eastern mountains and their momentum petered out. Meanwhile, the loss of so many units weakened resistance to the Russians as their tanks swept across the country's western plain; approximately 62,000 resisting troops under General Dome Sztojay were rounded up and defeated on a plateau near Debrecen on July 21, and when Russian troops approached the capital of Budapest, Horthy and his loyalists unenthusiastically let them enter the city unmolested, allowing them to proceed westward across the mountain passes (still held by Lakatos and his men) to push the Germans back to their own borders. By mid-August, the whole of the country was under Russian occupation, the Germans had been forced into retreat over yet another new front, and the Russians were poised to invade the former Czechoslovakia.
The push into eastern Slovakia was perhaps slightly more difficult than the Russian General Command anticipated. Anxious to remove any salient through which the Germans might attack east of the Vistula, Russian control over Hungary had not yet been fully consolidated when Zhukov's men in Gyor and Akhromeyev's men in Krakow embarked at the start of August to strangle the puppet “Slovak State”.
As it turned out, there was little need for such urgency. Slovakia had had a sizable German troop presence at the start of 1941, but most of the units had been sent to the Polish front in the spring, and after Hungary was occupied and Albatross was unceremoniously repulsed, the German High Command decided that the region was as good as lost and recalled most of their remaining troops to Bohemia. Therefore, there were few Germans left to oppose the invasion when it commenced, and the only real source of resistance came from the moribund security forces of the client state itself (whose authority and strength had been negligible even before the evacuating Germans stripped the land of most anything of military or strategic utility). The “Slovak Militia” was dealt with in short order, with many poorly-armed units simply surrendering immediately upon encountering Soviet troops. By the start of September, the Russian presence had fanned out through the region, the only serious opposition being nationalist partisans whose activities were little more than a recurring nuisance to the new occupiers.