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.

By the 14th century, the Holy Roman Empire was little more than a loose federation of the German princes who elected the Holy Roman Emperor. In 1438, Albert of Hapsburg became emperor, and for the next several centuries the Hapsburg line ruled the Holy Roman Empire until its decline in 1806. Relations between state and church were changed by the Reformation, which began with Martin Luther's 95 theses, and came to a head in 1547, when Charles V scattered the forces of the Protestant League at Mühlberg. The Counter-Reformation followed. A dispute over the succession to the Bohemian throne brought on the Thirty Years' War (1618–1648), which devastated Germany and left the empire divided into hundreds of small principalities virtually independent of the emperor.