Northern Transylvania is a region of Transylvania, situated within the territory of Romania. The population is largely composed of both ethnic Romanians and Hungarians, and the region has been part of Romania since 1918 (officially since 1920). During World War II, as a consequence of the territorial agreement known as the Second Vienna Award, the area was part of Hungary. After World War II, the Paris Peace Treaties returned Northern Transylvania to Romania.
Prior to World War I, Transylvania had been a semi-independent principality (Principality of Transylvania) under Ottoman sovereignty, a province (Principality/Grand Principality of Transylvania) of the Habsburg Monarchy/Austrian Empire, and part of the Kingdom of Hungary within the Austro-Hungarian Empire (from 1867 to 1918). The dual monarchy dissolved after the war. In December 1918, Transylvanian political organizations of ethnic Romanians and ethnic Hungarians each expressed loyalty to their respective homelands. The treaties of Saint-Germain (1919) and Trianon (1920) reflected the victory of the Allied Powers, granting Transylvania to Romania.
In June 1940, after Romania was forced (as a consequence of Molotov–Ribbentrop Pact) to settle a claim to Soviet Union over Bessarabian and Bukovinian territories, Hungary attempted to regain Transylvania, which it had lost in World War I. Germany and Italy pressured both Hungary and Romania to resolve the situation in a bilateral agreement. The two delegation met in Turnu Severin but the negotiations failed due to a demand for a 60,000 square kilometres territory on Hungarian side and only a population exchange on Romanian side. To impede a Hungarian-Romanian war in their "hinterland", the Axis powers pressured both governments to accept their arbitration: the Second Vienna Award.
Historian Keith Hitchins (Hitchins 1994) summarizes the situation created by the award:
- Far from settling matters, the Vienna Award had exacerbated relations between Romania and Hungary. It did not solve the nationality problem by separating all Magyars from all Romanians. Some 1,150,000 to 1,300,000 Romanians, or 48 per cent to over 50 per cent of the population of the ceded territory, depending upon whose statistics are used, remained north of the new frontier, while about 500,000 Magyars (other Hungarian estimates go as high as 800,000, Romanian as low as 363,000) continued to reside in the south.
According to the Hungarian census from 1941 the population of Northern Transylvania has dissimilar ratios, it counted 53.5% Hungarians and 39.1% Romanians. Hungary held Northern Transylvania from 1940 to 1944. Ethnic disturbances between Hungarians and Romanians continued during this period, with some Hungarians pursuing discrimination, harassment, or extreme violence against Romanians (see Treznea massacre, Ip massacre).
Like Jews living in Hungary, most of the Jews in Northern Transylvania (about 150,000) were sent to concentration camps during World War II, a move that was facilitated by local military and civilians. Some of the Romanian population in this region fled or was expelled, and the same happened with many Hungarians in Southern Transylvania. There was a mass exodus; over 100,000 people on both sides of the ethnic and political borders relocated.
The Second Vienna Award was voided by the Allied Commission through the Armistice Agreement with Romania (September 12, 1944) whose Article 19 stipulated the following: "The Allied Governments regard the decision of the Vienna award regarding Transylvania as and void and are agreed that Transylvania the greater part thereof) should be returned to Romania, subject to confirmation at the peace settlement, and the Soviet Government agrees that Soviet forces shall take part for this purpose in joint military operations with Romania against Germany and Hungary."
This came after King Michael's Coup following which Romania left the Axis and joined the Allies. Thus, the Romanian army fought Nazi Germany and its allies in Romania, regaining Northern Transylvania, and further on, in German occupied Hungary and Czechoslovakia (e.g. Budapest Offensive & Siege of Budapest and Prague Offensive).
- Károly Kocsis, Eszter Kocsisné Hodosi, Ethnic Geography of the Hungarian Minorities in the Carpathian Basin, Simon Publications LLC, 1998, p. 116 
- Hitchins, Keith (1994), Romania: 1866-1947, Oxford History of Modern Europe, Oxford University Press.