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 found in Mediaș, Moreşti, (Mureş County) and Ciumeşti. Expansion of Celtic groups in the area may be related to their invasion of the Balkans around 335 BC, when a massive colonization of the Tisa plain and the Transylvanian Plateau occurred following the death of Lysimachus. However, the eastward movement of the Celts into Transylvania used a different route from the one taken by the hordes that attacked the Balkans. Celts did not occupy all intra-Carpathian areas of Transylvania, stopping short of the Maramureş Depression for instance, where excavations have uncovered Dacian fortifications from the 4th and 3rd centuries BC. As