Hungary–Romania relations
During World War II, Hungary and Romania became allies and participated in the war against the Soviet Union. But after the coup on 23 August 1944 Romania switched sides and fought against Hungary. Consequently, Soviet and Romanian troops invaded Hungary, occupied Northern Transylvania by October 1944 and re-established the Romanian administration in the region in March 1945. The 1947 Treaty of Paris reaffirmed the borders between Romania and Hungary as originally defined in Treaty of Trianon 27 years earlier, thus confirming the return of Northern Transylvania to Romania.
Ion Antonescu
Carol's regime slowly dissolved into crisis, a dissolution accelerated after the start of World War II, when the military success of the core Axis Powers and the non-aggression pact signed by Germany and the Soviet Union saw Romania isolated and threatened ("see Romania during World War II"). In 1940, two of Romania's regions, Bessarabia and Northern Bukovina, were lost to a Soviet occupation consented to by the king. This came as Romania, exposed by the Fall of France, was seeking to align its policies with those of Germany. Ion Antonescu himself had come to value a pro-Axis alternative after the 1938 Munich Agreement, when Germany imposed demands on Czechoslovakia with the acquiescence of France and the United Kingdom, leaving locals to fear that, unless reoriented, Romania would follow. Angered by the territorial losses of 1940, General Antonescu sent Carol a general note of protest, and, as a result, was arrested and interned at Bistrița Monastery. While there, he commissioned Mihai Antonescu to establish contacts with Nazi German officials, promising to advance German economic interest, particularly in respect to the local oil industry, in exchange for endorsement. Commenting on Ion Antonescu's ambivalent stance, Hitler's minister to Romania, Wilhelm Fabricius, wrote to his superiors: "I am not convinced that he is a safe man."