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
In August 1914, when World War I broke out, Romania declared neutrality. Two years later, under the pressure of the Allies (especially France desperate to open a new front), on 14/27 August 1916 it joined the Allies, for which Romania was promised support for the accomplishment of national unity, including recognition of Romanian rights over Transylvania (which was at that time part of Austria-Hungary); Romania declared war on Austria-Hungary.