In classical antiquity, the central tribe which emerged in the territory of Kosovo were Dardani, who formed an independent polity known as the Kingdom of Dardania in the 4th century BCE. It was annexed by the Roman Empire by the 1st century BCE, and for the next millennium the territory remained part of the Byzantine Empire, whose rule was eroded by Slavic invasions beginning in the 6th–7th century AD. In the centuries thereafter control of the area alternated between the Byzantines and the First Bulgarian Empire. By the 13th century, Kosovo became the core of the of Serbian medieval state, and has also been the seat of the Serbian Orthodox Church from the 14th century, when its status was upgraded to a patriarchate. Ottoman expansion to the Balkans in the late 14th and 15th century lead to decline and fall of the Serbian Empire; the Battle of Kosovo of 1389 is considered to be one of the defining moments in Serbian medieval history. The Ottoman Empire ruled the area until the early 20th century.