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
Romania remained neutral for the first two years of World War I. Following the secret Treaty of Bucharest, according to which Romania would acquire territories with a majority of Romanian population from Austria-Hungary, it joined the Entente Powers and declared war on 27 August 1916. After initial advances the Romanian military campaign quickly turned disastrous for Romania as the Central Powers occupied two-thirds of the country within months, before reaching a stalemate in 1917. The October Revolution and Russian withdrawal from the War left Romania alone and surrounded, and a cease fire was negotiated at Focșani that December.