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
Principality of Transylvania (1570–1711) West and north Royal Hungary, Ottoman Hungary, and the Eastern Hungarian Kingdom under Ottoman suzerainty, which later became the Principality of Transylvania where Austrian and Turkish influences vied for supremacy for nearly two centuries. The Hungarian magnates of Transylvania resorted to policy of duplicity in order to preserve independence. Transylvania was administrated by Isabella, John Sigismund's mother from 1541 to 1551, when it fell for 5 years (1551–1556) under Habsburg rule. The House of Zapolya gained again the control of Transylvania in 1556, when the Diet of Szászsebes elected Sigismund as prince of Transylvania. Transylvania was now beyond the reach