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.
Southern Transylvania
Southern Transylvania was a region of the Kingdom of Romania between 1940 and 1944, during World War II. The region of Transylvania, belonging entirely to Romania when the war started in 1939, was split in 1940 between Romania and Hungary, with the latter taking Northern Transylvania in the aftermath of the Second Vienna Award.