The earliest inhabitants of Belgium were Celtic tribes, who were later conquered by the Romans in the 1st century BC. The Roman Empire introduced Christianity to the region, and after the fall of the empire, Belgium was ruled by a succession of Germanic tribes, including the Franks, who founded the Kingdom of France in the 5th century AD. In the 9th century, the Treaty of Verdun divided the Frankish Empire into three parts, with the western part becoming the Kingdom of France, the eastern part becoming the Holy Roman Empire, and the middle part becoming Lotharingia. Lotharingia was later divided into two parts, with the northern part becoming the Duchy of Lower Lorraine and the southern part becoming the Duchy of Upper Lorraine. In the 14th century, the Duchy of Lower Lorraine was divided into two parts, with the northern part becoming the County of Flanders and the southern part becoming the Duchy of Brabant. In the 15th century, the County of Flanders and the Duchy of Brabant were united to form the Burgundian Netherlands. In the 16th century, the Burgundian Netherlands was inherited by the Habsburgs, who ruled the region until the 18th century. In the 18th century, the Austrian Netherlands was conquered by France, and after the defeat of Napoleon in 1815, Belgium became part of the United Kingdom of the Netherlands. In 1830, Belgium gained its independence from the Netherlands and became a separate kingdom.