Ancient Thrace was an extensive region that occupied part of Southeastern Europe during the 1st millennium BC, before it was incorporated into the Roman Empire in the period from the late 1st century BC to the early 2nd century AD. Thrace was not an ethnically and culturally homogeneous region and ancient Thracians did never form a single nation and a unified state, although during the late 5th and first half of the 4th centuries BC the Odrysian Kingdom controlled the main part of the Thracian territory. In fact, ‘Thracians' was a general and collective ethnic name, used by the Greek and Roman authors when they were describing various events and indigenous people living in the region. Although the scholars are heavily restricted by the lack of domestic historical sources and Thracian literature, numerous different Thracian tribal names are well attested in the ancient Greek and Roman records. Simultaneously, other ethnic groups penetrated and settled in ancient Thrace and often they mingled with the local tribes. Thus, during the 1st millennium BC, the western periphery of Thrace was an interaction zone between Illyrians and Thracians, while the Scythians and the Thracians interacted actively in the northeastern areas of Thrace. The most significant historical event however, that influenced on the local tribes, was the Greek colonization of the Northern Aegean and Western Black Sea littorals that began in the middle of the 8th century BC and lasted through the following several centuries. Often, the ancient Greeks mingled with Thracians in the newly established colonies. As a result, certain local tribes gradually adopted many elements of the Classical Civilization and from the 6th to 4th centuries BC the Thracian tribal elite became partially Hellenized, while the ancient Greek became the official language of the diplomacy and administration. Another important event was the Persian occupation of Aegean Thrace during the late 6th and early 5th centuries BC, when this region became part of the Achaemenid Empire. The Persian presence not only influenced on the Thracian gold- and silverwork, but stimulated the process of state formation among the local Thracians and the powerful Odrysian Kingdom emerged after the withdrawal of the Persians. Later, during the reign of Philip of Macedon and Alexander the Great, almost entire Thrace was subdued and became part of the Macedonian Empire, while numerous Thracian aristocrats and warriors participated in the Eastern military campaigns of Alexander. During the early 3rd century BC, the Celts invaded ancient Thrace and many of them settled in the region and established a Celtic Kingdom that existed until the end of the century when it was liquidated by the Thracians, although some Celtic tribes continued to live in Thrace. The ancient historical sources regarding the last several centuries of the 1st millennium BC inform us of intermingled Thracians, Illyrians, Celts and Scythians, coexisting together in some regions of Thrace. Simultaneously, the Thracian culture adopted many Celtic elements, especially in the northern areas of Thrace.