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.
History of Romania
On 20 August 1944 the Soviet Red Army crossed the border into Romania. On 23 August 1944 Antonescu was toppled and arrested by King Michael I of Romania, who joined the Allies and declared war on Germany. On 31 August 1944 the Soviet Red Army entered Bucharest. Despite Romania's change of sides its role in the defeat of Nazi Germany was not recognized by the Paris Peace Conference of 1947.