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
During the Second World War, Romania tried again to remain neutral, but on 28 June 1940, it received a Soviet ultimatum with an implied threat of invasion in the event of non-compliance. Under pressure from Moscow and Berlin, the Romanian administration and the army were forced to retreat from Bessarabia as well from Northern Bukovina to avoid war. This, in combination with other factors, prompted the government to join the Axis. Thereafter, southern Dobruja was awarded to Bulgaria, while Hungary received Northern Transylvania as result of an Axis arbitration.
In 1940, Romania lost territory in both east and west: In June 1940, after receiving an ultimatum from the Soviet Union, Romania ceded Bessarabia and northern Bukovina ("see Soviet occupation of Bessarabia"). Two thirds of Bessarabia were combined with a small part of the USSR to form the Moldavian SSR. Northern Bukovina and Budjak were apportioned to the Ukrainian SSR. In August 1940, Northern Transylvania was awarded to Hungary by Germany and Italy through the Second Vienna Award. Southern Dobruja was also lost to Bulgaria shortly after Carol's abdication.