The Duchy of Saxony (Low German: Hartogdom Sassen) was a medieval duchy that encompassed Old Saxony, a historical region settled by the Saxons since the Migration Period. Initially independent, it came under the sphere of influence of the neighboring Frankish rulers as early as the Merovingian era. It was later subdued by Charlemagne during the Saxon Wars (772–804), and incorporated into the Carolingian Empire. From the Treaty of Verdun (843) onwards, it belonged to the East Frankish Kingdom, and thus became one of the five stem duchies of the emerging German realm. Saxon duke Henry the Fowler was elected German king in 919, thus founding the Saxon dynasty that ruled Germany and the Holy Roman Empire up to 1024.
From the middle of the 10th century, the Duchy was administered and then ruled by the House of Billung, and later by the House of Welf. Dukes of Saxony were also counted among the prince-electors. Upon the deposition of duke Henry the Lion in 1180, the ducal title, together with the electoral rights, fell to the House of Ascania. In time, many territories were split from the ducal jursidiction, such as the Principality of Anhalt in 1218 and the Duchy of Brunswick-Lüneburg in 1235. In 1296, the remaining lands were divided between the Ascanian ducal branches of Saxe-Lauenburg and Saxe-Wittenberg. Their mutual dispute over the right of wielding the electoral office was resolved by the Golden Bull of 1356, granting the title of Electors of Saxony to dukes of Saxe-Wittenberg. Upon the extinction of that ducal line in 1422, the remaining Ascanian line of Saxe-Lauenburg claimed both the Saxe-Wittenberg ducal possessions and the electoral dignity. Those requests were denied by the emperor Sigismund, who granted the possessions and titles in question to the House of Wettin. Since that time, the Wettin aglomeration of lands in the middle Elbe regions (Saxe-Wittenberg, March of Meissen, Osterland and others) led to the emergence of Upper Saxony, centered on the Wettin Electorate of Saxony, while core territories of the former stem duchy in the old Saxony came to be known as Lower Saxony.
The Saxon stem duchy covered the greater part of present-day Northern Germany, including the modern German states (Länder) of Lower Saxony and Saxony-Anhalt up to the Elbe and Saale rivers in the east, the city-states of Bremen and Hamburg, the Westphalian part of North Rhine-Westphalia, and the Holstein region (Nordalbingia) of Schleswig-Holstein. In the late 12th century, Duke Henry the Lion also occupied the adjacent area of Mecklenburg (the former Billung March).
The Saxons were one of the most robust groups in the late tribal culture of the times, and eventually bequeathed their tribe's name to a variety of more and more modern geopolitical territories, such as Old Saxony (Altsachsen), Upper Saxony, the Electorate, the Prussian Province of Saxony (in present-day Saxony-Anhalt), and the Kingdom of Saxony, the latter corresponding with the German Free State of Saxony, which bears the name today, despite its territory not having been part of the medieval duchy (see map on the right).
According to the Res gestae saxonicae by tenth century chronicler Widukind of Corvey, the Saxons had arrived from Britannia at the coast of Land Hadeln in the Elbe-Weser Triangle, called by the Merovingian rulers of Francia to support the conquest of the Thuringian kingdom, a mirror of the English origin myth where Saxon tribes from the region, under the leadership of legendary brothers Hengist and Horsa, were invited to post-Roman Britannia (see Anglo-Saxon settlement of Britain) by Vortigern.
The Royal Frankish Annals mention a 743 Frankish campaign led by the Carolingian Mayor of the Palace Carloman against the Saxons, followed by a second expedition together with his brother Pepin the Short the next year. In 747 their rebellious brother Grifo allied with Saxon tribes and temporarily conquered the stem duchy of Bavaria. Pepin, Frankish king from 750, again invaded Saxony and subdued several Westphalian tribes until 758.
In 772, Pepin's son Charlemagne started the final conquest of the Saxon lands. Though his ongoing campaigns were successful, he had to deal with the fragmentation of the Saxon territories in Westphalian, Eastphalian, Angrian, and Nordalbingian tribes, demanding the conclusion of specific peace agreements with single tribes, which soon were to be broken by other clans. The Saxons devastated the Frankish stronghold at Eresburg; their leader (Herzog) Widukind refused to appear at the 777 diet at Paderborn, retired to Nordalbingia and afterwards led several uprisings against the occupants, avenged by Charlemagne at the Massacre of Verden in 782. Widukind allegedly had to pledge allegiance in 785, having himself baptised and becoming a Frankish count. Saxon uprisings continued until 804, when the whole stem duchy had been incorporated into the Carolingian Empire.
Afterwards, Saxony was ruled by Carolingian officials, e.g. Wala of Corbie (d. 836), a grandson of Charles Martel and cousin of the emperor, who in 811 fixed the Treaty of Heiligen with King Hemming of Denmark, defining the northern border of the Empire along the Eider River. Among the installed dukes were already nobles of Saxon descent, like Wala's successor Count Ekbert, husband of Saint Ida of Herzfeld, a close relative of Charlemagne.
Ida of Herzfeld may have been an ancestor of the Saxon count Liudolf (d. 866), who married Oda of Billung and ruled over a large territory along the Leine river in Eastphalia, where he and Bishop Altfrid of Hildesheim founded Gandersheim Abbey in 852. Liudolf became the progenitor of the Saxon ducal, royal and imperial Ottonian dynasty; nevertheless his descendance, especially his affiliation with late Duke Widukind, has not been conclusively established.
In 865, under the Division of Frankfurt, king Louis the German granted Saxony, together with (Franconia) to his son Louis the Younger, who succeeded his father as king in those regions (876), and ruled there until his death in 882, being married to Liutgard of Saxony, daughter of count Liudolf.
Subdued only a few decades earlier, the Saxons rose to one of the leading tribes in East Francia; it is however uncertain if the Ottonians already held the ducal title in the ninth century. Liudolf's elder son Bruno (Brun), progenitor of the Brunswick cadet branch of the Brunonen, was killed in a battle with invading Vikings under Godfrid in 880. He was succeeded by his younger brother Otto the Illustrious (d. 912), mentioned as dux in the contemporary annals of Hersfeld Abbey, which, however, seems to have been denied by the Frankish rulers. His position was strong enough to wed Hedwiga of the Babenberg, daughter of mighty Duke Henry of Franconia, princeps militiae of King Charles the Fat. As all of Hedwiga's brothers were killed in the Franconian Babenberg feud with the rivalling Conradines, Otto was able to adopt the strong position of his father-in-law and to evolve the united Saxon duchy under his rule.
In 911, the East Frankish Carolingian dynasty went extinct with the death of King Louis the Child, whereafter the dukes of Saxony, Swabia and Bavaria met at Forchheim to elect the Conradine duke Conrad I of Franconia king. One year later, Otto's son Henry the Fowler succeeded his father as Duke of Saxony. According to the medieval chronicler Widukind of Corvey, King Conrad designated Henry his heir, thereby denying the succession of his own brother Eberhard of Franconia, and in 919 the Saxon duke was elected King of East Francia by the assembled Saxon and Franconian princes at Fritzlar.
Henry was able to integrate the Saxon, Swabian, Bavarian and Lotharingian duchies into a united realm, vital to handle the continuous attacks by Hungarian forces. In 928/929, Henry's Saxon forces invaded regions across the Elbe river and subdued various tribes of the Polabian Slavs. Those campaigns led to the establishment of German strongholds in Brandenburg and Meissen, as well as the surrender of Duke Wenceslaus of Bohemia, thus marking the beginning of the German eastward expansion (Ostsiedlung). Slavic tribes on the borders of Eastphalia, such as Drevani, who lived along Elbe river in the region later known as Altmark, were gradually subdued and integrated, thus expanding Saxon frontiers towards east.