Transylvania as originally defined in Treaty of Trianon, 27 years earlier, thus confirming the return of Northern Transylvania to Romania. From 1947 to 1989, Transylvania, along with the rest of Romania, was under a communist regime. The ethnic clashes of Târgu Mureș occurred between ethnic Romanians and Hungarians in March 1990 after the fall of the communist regime and became most notable inter-ethnic incident in the post-communist era. The Transylvanian Plateau, high, is drained by the Mureș, Someș, Criș, and Olt rivers, as well as other tributaries of the Danube. This core of historical Transylvania roughly corresponds with nine counties of
Transylvania 1526, Transylvania was a voivodeship in the Kingdom of Hungary, led by a voivode appointed by the King of Hungary. After the Battle of Mohács in 1526, Transylvania became part of the Kingdom of János Szapolyai. Later, in 1570, the kingdom transformed into the Principality of Transylvania, which was ruled primarily by Calvinist Hungarian princes. During that time, the ethnic composition of Transylvania transformed from an estimated near equal number of the ethnic groups to a Romanian majority. Vasile Lupu estimates their number already more than one-third of the population of Transylvania in a letter to the sultan around 1650.