Transylvania was indeed part of Hungary, although it was joined with Romania due to political changes in the wake of First World War. According to the 1910 census in Transylvania, the region has been inhabited chiefly by Romanians, who constituted 54% of the population, with Hungarians forming 29%, with Germans contributing roughly 11%. It should be noted however, that the Romanian identity is largely a result of the 18th-century societal changes, primarily the rise of national ideologies that led to the formation of nation states in 19th and 20th century, although the idea of Roman origins dates back to works of Olah Miklos, mainly Hungaria, written in 1536. Until then, people who are identified as Romanians and who shared language and principal cultural traits were generally called 'Wallachians', even if they lived outside the Principality of Wallachia.

Hungarians attained control of Transylvania in late 10th century, although colonization of these territories was limited. This changed after the Mongol invasions in 1th century, when during the process of reconstruction, Hungarian landowners began inviting large numbers of Germans, chiefly to newly located towns and cities. This caused ethnic divisions to reflect the socio-economic ones, with Hungarians dominating the landed elite and clergy, Wallachians being the peasant majority and Germans being the principal ethnic group in cities. Since early 16th century, tension between Habsburgs, Ottomans Empire and Polish-Lithuanian Commonwealth (as well as between Transylvania, Moldavia and Wallachia, seeking support in all three aforementioned powers) resulted in Transylvania becoming a territory dependent on Ottoman Empire. It is worth noting that in year 1600, Michael the Brave managed to unify Transylvania, Moldavia and Wallachia, creating a country more or less corresponding to present borders of Romania, although it fell apart few months later due to the opposition from virtually all major powers in the region trying to prevent a new, strong competition from appearing. Transylvania has been annexed by Habsburgs few decades later, after the Battle of Vienna in 1683, wth this territorial cession being made official in the Treaty of Karlowitz in 1699.

In 18th and 19th century, Wallachia, Moldavia and Transylvania attempted to shake the foreign dominance, but all these attempts failed. In 1859 Ioan Cuza took power in Wallachia and Moldavia (as Alexandru Ioan I), essentially joining these two regions in personal union, leading to the official recognition of the Duchy of Romania in 1862. Although still under the Ottoman control, formation of this state greatly bolstered the nationalist and independence stance of the Romanians as it increased the chances to gain sovereignty. And indeed, when the war between Russian and Ottoman Empires ended with the defeat of the latter, Duchy of Romania, a Russian ally, has been officially recognized as a sovereign state on the Congress of Berlin in 1878. Concurrently, in the wake of the Spring of Nation, aforementioned Romanian national identity was growing, along with the idea of merging all the people speaking Wallachian language(s) within one country.

Now, during the Great War, Russian government made a proposition to Ferdinand I, king of Romania, offering the arbitration allowing the merge of Transylvania with the Romania in the case Central Powers, including Austro-Hungary are defeated. Ferdinand I accepted that offer and eventually declared war on the side of Entente. His attempt to annex Transylvania by force ended in complete failure, when Romanian forces entering Transylvania were dispersed by the German army. This defeat has been exacerbated by the October Revolution in was followed by the armistice and then the withdrawal of Soviet Russia from the war through the peace treaty of Brest-Litovsk signed on 3rd March 1918. Having lost their only ally in the region, Romania was forced to accept ultimatum of Central Powers and sign preliminary peace agreements two days later. Peace treaty between Romania and Central Powers has eventually been signed on 7th May 1918 in Bucharest and granted Besarabia to Romania, but also forced the latter to cede Dobrudja to Bulgaria, and several strategic mountain passes to Austro-Hungary. Furthermore, it has been forced to accept economic concessions to German, Austrian and Hungarian businesses, especially in the oil and mining industries. The treaty, although unfavourable to Romania, has not been signed by king. This became handy in late September that year, when the succesful operations forced Bulgaria to sign armistice with the Entente. This propelled Romanian government to treat Bucharest treaty as void and make another attempt to take Transylvania.

The war finally ended on 11th November 1918, and after the defeat of Austro-Hungary and Germany, Romanian Committee operating in the latter proposed the annexation of the Hungarian territories inhabited by Romanian majority by Romania. Great National Assembly that gathered in Alba Iulia voted for this idea on 1st December 1918, and with the Proclamation of the Union, Transylvania joined the Kingdom of Romania. The October Revolution and fall of Central Powers resulted in serious political upheaval in Europe, formalized by treaty of Versailles and other treaties detailing the changes in the political landscape. In this case, the pivotal agreement was the Treaty of Trianon signed on 4th June 1920 that divided the territory of Hungary. Its northern part became eastern part of Czechoslovakia (now Slovakia), southwestern area entered the borders of Yugoslavia, small territory in the West (Burgenland) and in the north (Spish and Orava) were granted to Austria and Poland respectively and the entire Transylvania with parts of Banat, Maramaros and Körösvidék has been formally recognized as part of Romania.

Thus, due to the emergence of Romanian national identity, the attempts to unify all the Wallachian/Romanian speaking areas were undertaken, and between the Austro-Hungary being on the losing side of the Great War, the tendency to assist the self-determination of the nations and general support of union among the Romanian majority in Transylvania and Romania, the former has been unified with the latter in 1918, what has been recognized by European powers in 1920.