Lothair I or Lothar I (Dutch and Medieval Latin: Lotharius; German: Lothar; French: Lothaire; Italian: Lotario) (795 29 September 855) was emperor (817855, co-ruling with his father until 840), and the governor of Bavaria (815817), King of Italy (818855) and Middle Francia (840855).

Lothair was the eldest son of the Carolingian emperor Louis the Pious and his wife Ermengarde of Hesbaye, daughter of Ingerman the duke of Hesbaye. On several occasions, Lothair led his full-brothers Pepin I of Aquitaine and Louis the German in revolt against their father to protest against attempts to make their half-brother Charles the Bald a co-heir to the Frankish domains. Upon the father's death, Charles and Louis joined forces against Lothair in a three-year civil war (840843). The struggles between the brothers led directly to the breakup of the Frankish Empire assembled by their grandfather Charlemagne, and laid the foundation for the development of modern France and Germany.

Louis the Pious (German: Ludwig der Fromme; 16 April 778 – 20 June 840), also called the Fair, and the Debonaire, was King of the Franks and co-emperor with his father, Charlemagne, from 813. He was also King of Aquitaine from 781. As the only surviving son of Charlemagne and Hildegard, he became the sole ruler of the Franks after his father's death in 814, a position which he held until his death, save for the period 833–34, during which he was deposed.

During his reign in Aquitaine, Louis was charged with the defence of the empire's southwestern frontier. He conquered Barcelona from the Emirate of Córdoba in 801 and asserted Frankish authority over Pamplona and the Basques south of the Pyrenees in 812. As emperor he included his adult sons, Lothair, Pepin, and Louis, in the government and sought to establish a suitable division of the realm among them. The first decade of his reign was characterised by several tragedies and embarrassments, notably the brutal treatment of his nephew Bernard of Italy, for which Louis atoned in a public act of self-debasement.

In the 830s his empire was torn by civil war between his sons, only exacerbated by Louis's attempts to include his son Charles by his second wife in the succession plans. Though his reign ended on a high note, with order largely restored to his empire, it was followed by three years of civil war. Louis is generally compared unfavourably to his father, though the problems he faced were of a distinctly different sort.