which were also culturally different. The border between the two halves has been identified as the Haemus Mountains or the Danube slightly further north. Southern Thrace covered the fertile valley between the Haemus and the Rhodopes, the Strandzha and the shores of the Propontis and the Aegean and Black Seas. The western boundary was marked by the Strymon and the upper Morava. Northern Thrace was defined by the Danube, the Carpathians and the adjacent western tip of the Pontic–Caspian steppe, thus enclosing parts of the territory now comprising modern Romania, Moldavia, Serbia and Ukraine. Thrace also extended into what is now northwestern Turkey both west and east (Bithynia, Mysia) of the Propontis. Thracians had already settled in the 2nd millennium BCE, and were featured in the epics of Homer. Occasional references to them appear in the following centuries, although it was not until the 5th century when Greek literature developed an interest in discussing non-Greeks more extensively. In the 7th and 6th centuries, much of the Thracian coast was settled by Greek colonists who founded numerous towns, like Thasos, Byzantion or Odessos. The political history of the Thracian tribes of this age is virtually unknown, although it is recorded that in the late 6th century, Athenian settlers interacted with a "king of Thrace" (and possible predecessor of the Odrysian kings?) residing north of the Chersonese peninsula. The absence of imported artefacts confirms that inland Thrace north of the Rhodopes remained largely isolated from the Aegean trade until the late 6th century. Persian Thrace In around 513 BC, an army of the mighty Persian dynasty of the Achaemenids crossed the Bosphorus, after already having subdued the Thracians of Bithynia thirty years earlier. King Darius I's goal was a punitive expedition against the Scythians at the northern shores of the Black Sea. Most eastern Thracian tribes submitted peacefully, except of the Getae, who were defeated. More expeditions under the generals Megabazus and Mardonius as well as king Xerxes I followed, even though they only managed to secure the Aegean coast. It seems most likey that the Achaemenids did not establish a satrapy (provincial administration) in Thrace, even though the historian Herodotus claimed that the subdued regions had to pay taxes. As a matter of fact, there is no evidence for important administrative centers. Instead, Persian authority was merely exercised through a couple of garrisoned forts, most importantly those of Doriskos and Eion.