Lothair I

The eldest son of Emperor Louis the Pious, Lothair I was associated with the imperial office early in life. The Ordinatio Imperii of 817 named him co-emperor and principal heir, granting him precedence over his younger brothers Pepin of Aquitaine and Louis the German. He was sent to govern Italy and crowned emperor in Rome by Pope Paschal I in 823, establishing the Italian kingdom as the base of his power for much of his career.

The birth of a fourth son, Charles, to Louis the Pious and his second wife Judith in 823 unsettled the succession arrangement and drew Lothair into open conflict with his father. He took a leading part in the rebellions of 830 and 833, during the second of which Louis the Pious was briefly deposed. After the emperor's restoration, Lothair was largely confined to Italy, where he remained until his father's death in 840.

Claiming authority over the whole empire as senior emperor, Lothair was opposed by his brothers Louis the German and Charles the Bald, who defeated him at Fontenoy in 841 and sealed their alliance with the Strasbourg Oaths the following year. The Treaty of Verdun in 843 ended the civil war by dividing the Frankish realm in three. Lothair retained the imperial title and the middle kingdom, an elongated territory running from Frisia and the Low Countries through Burgundy and Provence into Italy, and including both imperial centers, Aachen and Rome. The arrangement preserved his dignity but left him a realm difficult to defend and lacking internal cohesion.

Lothair had married Ermengarde, daughter of Hugh, count of Tours, in 821, and the couple had several children. Shortly before his death he divided the middle kingdom among his three sons in the Partition of Prüm of 855: Louis II received Italy and the imperial title, Lothair II the northern lands later called Lotharingia, and Charles received Provence. Gravely ill, Lothair entered the monastery of Prüm as a monk and died there within days, in September 855. The rapid fragmentation of his kingdom after his death made the middle realm an enduring object of contention between its eastern and western neighbors.