The Celts are believed to have been the first inhabitants of Germany. They were followed by German tribes at the end of the 2nd century B.C. German invasions destroyed the declining Roman Empire in the 4th and 5th centuries A.D. One of the tribes, the Franks, attained supremacy in western Europe under Charlemagne, who was crowned Holy Roman Emperor in 800. By the Treaty of Verdun (843), Charlemagne's lands east of the Rhine were ceded to the German Prince Louis. Additional territory acquired by the Treaty of Mersen (870) gave Germany approximately the area it maintained throughout the Middle Ages. For several centuries after Otto the Great was crowned king in 936, German rulers were also usually heads of the Holy Roman Empire.