Why did Germany occupy Hungary in March 1944?

Germany occupied Hungary in March 1944 primarily because Adolf Hitler feared that the Hungarian government was becoming an unreliable ally as the Soviet Union approached the Carpathians.12

Under Prime Minister Miklós Kállay, Hungary had been conducting a delicate balancing act. While officially part of the Axis powers, Kállay was actively seeking to extricate the country from the war by putting out peace feelers to the Western Allies. Hitler, who was aware of these secret activities, decided he could not risk leaving his vital communications lines in the Danube valley at the mercy of a regime he deemed untrustworthy.12

On March 19, 1944, while Miklós Horthy was visiting Hitler in Klessheim, Germany, the occupation began. Hitler presented Horthy with an ultimatum: accept full cooperation under German supervision or face an undisguised occupation with the harsh treatment accorded to an enemy. This forced Horthy to appoint a collaborationist government, effectively ending Hungary's attempts to negotiate a separate peace at that time.12