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.

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.

In 814, the elderly emperor died, and left his sole surviving legitimate son Louis the Pious as successor to his vast empire.

In November 824, Lothair promulgated a statute, the Constitutio Romana, concerning the relations of pope and emperor, which reserved the supreme power to the secular potentate, and he afterwards issued various ordinances for the good government of Italy.

[1] On Lothair's return to his father's court, his stepmother Judith won his consent to her plan for securing a kingdom for her son Charles, a scheme which was carried out in 829,[1] when the young prince was given Alemannia as king.

[citation needed] Lothair, however, soon changed his attitude and spent the succeeding decade in constant strife over the division of the Empire with his father.

He was alternately master of the Empire, and banished and confined to Italy, at one time taking up arms in alliance with his brothers and at another fighting against them, whilst the bounds of his appointed kingdom were in turn extended and reduced.

[2] A decisive battle was fought at Fontenay-en-Puisaye on 25 June 841, when, in spite of his[1] and his allied nephew Pepin II of Aquitaine's[citation needed] personal gallantry, Lothair was defeated and fled to Aachen.

[6][7] In 855 he became seriously ill, and despairing of recovery renounced the throne, divided his lands among his three sons, and on 23 September entered the monastery of Prüm, where he died six days later.