Who was the last ruler to maintain the unity of the Carolingian Empire?

Louis I (778-840), or Louis the Pious, was king of the Franks and emperor of the West from 814 to 840. The son and successor of Charlemagne, he was the last ruler to maintain the unity of the Carolingian Empire.

Louis conquered Barcelona from the Muslims 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 characterized by several tragedies and embarrassments, notably the brutal treatment of his nephew Bernard of Italy.

The Carolingian Empire (800–888), also known as the Empire of the Romans and Franks, was a large Frankish-dominated empire in western and central Europe during the early Middle Ages. It was ruled by the Carolingian dynasty, which had ruled as kings of the Franks since 751 and as kings of the Lombards in Italy from 774. In 800, the Frankish king Charlemagne was crowned emperor in Rome by Pope Leo III in an effort to revive the Roman Empire in the west. The Carolingian Empire is considered the first phase in the history of the Holy Roman Empire, which lasted until 1806.

After a civil war (840–43) following the death of Emperor Louis the Pious, the empire was divided into autonomous kingdoms, with one king still recognised as emperor, but with little authority outside his own kingdom.