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.
Romania in World War II
In a radio broadcast to the Romanian nation and army on the night of 23 August King Michael issued a cease-fire, proclaimed Romania's loyalty to the Allies, announced the acceptance of an armistice (to be signed on September 12) offered by Great Britain, the United States, and the USSR, and declared war on Germany. The coup accelerated the Red Army's advance into Romania, but did not avert a rapid Soviet occupation and capture of about 130,000 Romanian soldiers, who were transported to the Soviet Union, where many perished in prison camps. The armistice was signed three weeks later on 12 September 1944, on terms virtually dictated by the Soviet Union. Under the terms of the armistice, Romania announced its unconditional surrender to the USSR and was placed under occupation of the Allied forces with the Soviet Union as their representative, in control of media, communication, post, and civil administration behind the front. Some attribute the postponement of a formal Allied recognition of the "de facto" change of orientation until 12 September (the date the armistice was signed in Moscow) to the complexities of the negotiations between the USSR and UK.