Lothair I (795-855), also known as Lothar I, was the eldest son of Emperor Louis the Pious and grew up in the court of his grandfather, Emperor Charlemagne. When Louis became sole emperor in 814, he sent Lothair to govern Bavaria in 815. Lothair was crowned as co-emperor and declared as principal heir in 817, and would be overlord to his younger brothers, Pippin of Aquitaine and Louis the German, as well as his cousin Bernard of Italy.

When his father died in 840, Lothair ignored all previous plans for partitioning and claimed the whole of the Holy Roman Empire for himself, leading to another civil war which lasted around three years.