Western Thrace or West Thrace (, [Dytikí] Thráki ; ; , Zapadna/Belomorska Trakiya), also known as Greek Thrace, is a geographic and historical region of Greece, between the Nestos and Evros rivers in the northeast of the country; East Thrace, which lies east of the river Evros, forms the European part of Turkey, and the area to the north, in Bulgaria, is known as Northern Thrace. Inhabited since paleolithic times, it has been under the political, cultural and linguistic influence of the Greek world since the classical era; Greeks from the Aegean islands extensively colonized the region (especially the coastal part) and built prosperous cities such as Abdera (home of Democritus, the 5th-century B.C. philosopher who developed an atomic particle theory, and of Protagoras, a leading sophist) and Sale (near present-day Alexandroupolis). Under the Byzantine Empire, Western Thrace benefited from its position close to the imperial heartland and became a center of medieval Greek commerce and culture; later, under the Ottoman Empire, a number of Muslims settled there, marking the birth of the Muslim minority of Greece. Topographically, Thrace alternates between mountain-enclosed basins of varying size and deeply cut river valleys. It is divided into the three regional units (former prefectures): Xanthi, Rhodope and Evros, which together with the Macedonian regional units of Drama, Kavala and Thasos form the Region of East Macedonia and Thrace. The Fourth Army Corps of the Hellenic Army has its headquarters in Xanthi; in recent years, the region has attracted international media attention after becoming a key entering point for illegal immigrants trying to enter European Union territory; Greek security forces, working together with Frontex, are also extensively deployed in the Greco-Turkish land border. Demographics The approximate area of Western Thrace is 8,578 km² with a population of 371,208 according to the 2011 census. It's estimated that two-thirds (67%) of the population are Orthodox Christian Greeks, while about a third (33%) are Muslims who are an officially recognised minority of Greece. Of these, about a quarter are of Turkish origin, while another quarter are Pomaks who mainly inhabit the mountainous parts of the region. The rest are Muslim Greeks or Romani. The Romani of Thrace are also mainly Muslim, unlike their ethnic kin in other parts of the country who generally profess the Orthodox faith of the Greek majority. Thrace is bordered by Bulgaria to the north, Turkey to the east, the Aegean Sea (Greece)