Northern Transylvania

Northern Transylvania (, ) was the region of the Kingdom of Romania that during World War II, as a consequence of the August 1940 territorial agreement known as the Second Vienna Award, became part of the Kingdom of Hungary. With an area of , the population was largely composed of both ethnic Romanians and Hungarians. 
In October 1944, Soviet and Romanian forces gained control of the territory, and by March 1945 Northern Transylvania returned to Romanian administration. After the war, this was confirmed by the Paris Peace Treaties of 1947.
Background.
The region has a varied history. It was once the nucleus of the Kingdom of Dacia (82 BC–106 AD). In 106 AD the Roman Empire conquered the territory, systematically exploiting its resources. After the Roman legions withdrew in 271 AD, it was overrun by a succession of various tribes, bringing it under the control of the Carpi, Visigoths, Huns, Gepids, Avars, and Slavs. From the 9th to the 11th century, Bulgarians ruled Transylvania.
The Magyars conquered much of Central Europe at the end of the 9th century and for almost six hundred years, Transylvania was a voivodeship in the Kingdom of Hungary. After the Battle of Mohács in 1526 and the Hungarian defeat by the Ottomans, Transylvania became a semi-independent principality (the Principality of Transylvania) under local Hungarian nobility rule, but owing suzerainty to the Ottoman Empire. It then became a province (Principality/Grand Principality of Transylvania) of the Habsburg Monarchy/Austrian Empire as a Land of the Hungarian Crown, and after 1848, and again from 1867 to 1918 it was incorporated into the Kingdom of Hungary within the Austro-Hungarian Empire. The dual monarchy dissolved after World War I.
The ethnic Romanians, who formed the majority population of Transylvania, elected representatives who proclaimed the Union with Romania on 1 December 1918. The Proclamation of Union at Alba Iulia was adopted by the Deputies of the Romanians from Transylvania, supported one month later by the vote of the Deputies of the Transylvanian Saxons. By Spring 1919, during the Hungarian–Romanian War, Transylvania came under administrative control of Romania. Eventually in June 1920 the Treaty of Trianon assigned Transylvania to the Kingdom of Romania.
Second Vienna Award.
In June 1940, Romania was forced (as a consequence of Molotov–Ribbentrop Pact) to submit to a Soviet ultimatum and accept the annexation by the Soviet Union of Bessarabia and Northern Bukovina. Subsequently, Hungary attempted to regain Transylvania, which it had lost in the immediate aftermath of World War I. Germany and Italy pressured both Hungary and Romania to resolve the situation in a bilateral agreement. The two delegations met in Turnu Severin on August 16, but the negotiations failed due to a demand for a territory from the Hungarian side and only an offer of population exchange from the 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, signed on August 30.
Historian Keith Hitchins summarizes the situation created by the award:
The Hungarian population was in the unusual situation of being an overwhelming majority in an area of southeastern Transylvania, deep within Romania and far from the Hungarian border (the area, known as Székely Land, is today mainly in Harghita, Covasna, and Mureș counties), and not simply only in certain areas next to the Hungarian border as in the case of Czechoslovakia and Bačka or Baranya. The solution decided upon was to gouge a claw-shaped corridor through northwestern Romania, including a large Romanian-populated area, in order to incorporate this Hungarian-majority area within Hungary.
Population of Northern Transylvania, as per 1930 Romanian census:
Before the arbitration, in 1940, according to the Romanian estimates, in Northern Transylvania there were 1,304,903 Romanians (50.2%) and 978,074 (37.1%) Hungarians. One year later, after the arbitration, according to the Hungarian census, the population of Northern Transylvania had dissimilar ratios, it counted 53.5% Hungarians and 39.1% Romanians.
Hungarian rule.
Hungary held Northern Transylvania from September 1940 to October 1944. In 1940 ethnic disturbances between Hungarians and Romanians continued after some incidents following the entrance of the Hungarian Army, culminating in massacres at Treznea and Ip. 
After some ethnic Hungarian groups considered unreliable or insecure were sacked/expelled from Southern Transylvania, the Hungarian officials also regularly expelled some Romanian groups from Northern Transylvania. Also, many Hungarians and Romanians fled or chose to opt between the two countries. There was a mass exodus; over 100,000 people on both sides of the ethnic and political borders relocated. This continued until 1944.
On March 19, 1944, following the occupation of Hungary by the Nazi Germany army through Operation Margarethe, Northern Transylvania came under German military occupation. Like the 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. Following several decrees of the Hungarian government and high-level consultations at a meeting on April 26 with László Endre in Szatmárnémeti (now Satu Mare), the deportation of the Jews was decided. On May 3, authorities in Dés (now Dej) launched the action of ghettoization of Jews in the Bungăr forest, where 3,700 Jews from Dej and 4,100 Jews from other localities in the area were imprisoned. During the operation of the Dej ghetto, Jews were mistreated, tortured, and starved. The deportation of the Jews to the Nazi death camps was done with freight wagons, in three stages: the first transport on May 28 when 3,150 Jews were deported; the second on June 6, when 3,360 Jews were deported; the third on June 8, when the last 1,364 Jews were deported. Most of those deported were exterminated in the Auschwitz–Birkenau camp, with just over 800 deportees surviving. The Kolozsvár Ghetto (in what is now Cluj-Napoca) was initiated on May 3, and was put under the command of László Urbán, the local police chief. The ghetto, comprising about 18,000 Jews, was liquidated in six transports to Auschwitz, with the first deportation occurring on May 25, and the last one on June 9. Other ghettoes that were set up in Northern Transylvania during this period were the Oradea ghetto (the largest one, with 35,000 Jews), the Baia Mare ghetto, the Bistrița ghetto, the Cehei ghetto, the Reghin ghetto, the Satu Mare ghetto, and the Sfântu Gheorghe ghetto.
After King Michael's Coup of August 23, 1944, 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 in Slovakia and Protectorate of Bohemia and Moravia, for instance, in the Budapest Offensive, the Siege of Budapest, and the Prague Offensive.
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 null and void and are agreed that Transylvania (or 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."
The territory was occupied by the Allied forces by late October 1944. On October 25, at the Battle of Carei, units of the Romanian 4th Army under the command of General Gheorghe Avramescu took control of the last piece of the territory ceded in 1940 to Hungary. However, due to the activities of Romanian paramilitary forces, the Soviets expelled the Romanian administration from Northern Transylvania in November 1944 and did not allow them to return until March 1945.
On 20 January 1945, Hungary accepted the obligation to evacuate all Hungarian troops and officials from the territory, to retreat to its pre-war borders, and to repeal all legislative and administrative regulations in connection with the incorporation of the territory.
The 1947 Paris Peace Treaty reaffirmed the borders between Romania and Hungary, as originally defined in the Treaty of Trianon, 27 years earlier, thus confirming the return of Northern Transylvania to Romania.
Geography.
Northern Transylvania is a diverse region, both in terms of landscape and population. It contains both largely rural areas (such as Bistrița-Năsăud County) as well as major cities, such as Cluj-Napoca, Oradea, Târgu Mureș, Baia Mare, and Satu Mare. Centers of Hungarian culture, such as Miercurea Ciuc and Sfântu Gheorghe, are also part of the region. An important tourist destination is Maramureș County, an area known for its beautiful rural scenery, local small woodwork, including wooden churches, its craftwork industry, and its original rural architecture.