The Saxons long resisted both Xianity and the Franks. However in 768 Charlemagne (Carolus Magnus, "Charles the Great") succeeded his father Pippin III and became the second Carolingian king, and he devoted a long series of annual campaigns, the Saxon Wars (772 – 804), to grinding them down. In 772 Charlemagne occupied the Saxon stronghold at the Eresburg and had the Irminsûl cut down. In 773 Charlemagne went to Italy and the Saxons reoccupied the Eresburg, but he took it back the following year. In 778 Charlemagne campaigned in Hispania and the Saxons advanced to Deutz on the Rhine and plundered along the river. In 782 Charlemagne conducted a Blutgericht ("blood trial") at Verden on the Aller River, ordering more than 4,500 Saxon prisoners killed. Ultimately despite brave resistance by Duke Widukind, the Saxons were defeated. The survivors were forced to renounce their gods (we have the oath they were made to swear) and were baptized.

On Xmas Day, 800 CE, Charlemagne was crowned Emperor by Pope Leo III to counter the power of the Byzantine Empire, beginning what is known as the Holy Roman Empire.

Under Carolingian rule, the Saxons were reduced to tributary status. There is evidence that they, as well as Slavic tributaries such as the Abodrites and the Wends, often provided troops to their Carolingian overlords. The Dukes of Saxony became kings (Henry I, the Fowler, 919) and later the first emperors (Henry's son, Otto I, the Great) of Germany during the 10th century, but they lost this position in 1024. The duchy was divided up in 1180 when Duke Henry the Lion, Emperor Otto's grandson, refused to follow his cousin, Emperor Frederick Barbarossa, into war in Lombardy.