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.
Romanians
During the 1870s, the United Romanian Principalities (then led by Hohenzollern-Sigmaringen Domnitor Carol I) fought a War of Independence against the Ottomans, with Romania's independence being formally recognised in 1878 at the Treaty of Berlin. Although the newly founded Kingdom of Romania initially allied with Austria-Hungary, Romania refused to enter World War I on the side of the Central Powers, because it was obliged to wage war only if Austria-Hungary was attacked. In 1916, Romania joined the war on the side of the Triple Entente.