The partition of the Carolingian Empire by the Treaty of Verdun in 843. From Histoire Et Géographie - Atlas Général Vidal-Lablache, 1898.
In August 843, after three years of civil war following the death of emperor Louis the Pious on 20 June 840, the Treaty of Verdun was signed by his three sons and heirs. The division of lands was largely based on the Meuse, Scheldt, Saone and Rhone rivers. While the eldest son Lothair I kept the imperial title and the kingdom of Middle Francia, Charles the Bald received West Francia and Louis the German received the eastern portion of mostly Germanic-speaking lands of Duchy of Saxony, Austrasia, Alamannia, Duchy of Bavaria, and March of Carinthia.
The contemporary East Frankish Annales Fuldenses describes the kingdom being "divided in three" and Louis "acceding to the eastern part". The West Frankish Annales Bertiniani describe the extent of Louis's lands: "at the assigning of portions, Louis obtained all the land beyond the Rhine river, but on this side of the Rhine also the cities of Speyer, Worms and Mainz with their counties". The kingdom of West Francia went to Louis's younger half-brother Charles the Bald and between their realms a kingdom of Middle Francia, incorporating Italy, was given to their elder brother, the Emperor Lothair I.
While Eastern Francia contained about a third of the traditional Frankish heartland of Austrasia, the rest consisted mostly of lands annexed to the Frankish empire between the fifth and the eighth century. These included the duchies of Alemannia, Bavaria, Saxony and Thuringia, as well as the northern and eastern marches with the Danes and Slavs. The contemporary chronicler Regino of Prüm wrote that the "different people" (diversae nationes populorum) of East Francia, mostly Germanic- and Slavic-speaking, could be "distinguished from each other by race, customs, language and laws" (genere moribus lingua legibus).
In 869 Lotharingia was divided between West and East Francia under the Treaty of Meersen. The short lived Middle Francia turned out to be the theatre of Franco-German wars up until the 20th century.
All the Frankish lands were briefly reunited by Charles the Fat, but in 888 he was deposed by nobles and in East Francia Arnulf of Carinthia was elected king.
The increasing weakness of royal power in East Francia meant that dukes of Bavaria, Swabia, Franconia, Saxony and Lotharingia turned from appointed nobles into hereditary rulers of their territories. Kings increasingly had to deal with regional rebellions.
In 911 Saxon, Franconian, Bavarian and Swabian nobles no longer followed the tradition of electing someone from the Carolingian dynasty as a king to rule over them and on November 10, 911 elected one of their own as the new king. Because Conrad I was one of the dukes, he found it very hard to establish his authority over them. Duke Henry of Saxony was in rebellion against Conrad I until 915 and struggle against Arnulf, Duke of Bavaria cost Conrad I his life. On his deathbed Conrad I chose Henry of Saxony as the most capable successor. This kingship changed from Franks to Saxons, who had suffered greatly during the conquests of Charlemagne. Henry, who was elected to kingship by only Saxons and Franconians at Fritzlar, had to subdue other dukes and concentrated on creating a state apparatus which was fully utilized by his son and successor Otto I. By his death in July 936 Henry had prevented collapse of royal power as was happening in West Francia and left a much stronger kingdom to his successor Otto I. After Otto I was crowned as the Emperor in Rome in 962 the era of the Holy Roman Empire began.