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
Celts in Transylvania area. The Celts exercised politico-military rule over Transylvania between the 4th and 2nd century BC and brought with them a more advanced iron-working technology. They were also responsible for the spread of the potter's wheel into a much wider area than the one they occupied. Large areas of ancient Dacia, which were populated early in the First Iron Age by Thracian people, were affected by a massive migration of Iranian Scythians moving east to west during the first half of the first millennium BC. They were followed by a second equally large wave of Celts migrating west to east. Celts