LOTHAIR I. (795-855), Roman emperor, was the eldest son of the emperor Louis I., and his wife Irmengarde. Little is known of his early life, which was probably passed at the court of his grandfather Charlemagne, until 815, when he became ruler of Bavaria. When Louis in 817 divided the empire between his sons, Lothair was crowned joint emperor at Aix-la-Chapelle and given a certain superiority over his brothers. In 821 he married Irmen garde (d. 851), daughter of Hugo, count of Tours; in 822 under took the government of Italy; and, on April 5, 823, was crowned emperor by Pope Paschal I. at Rome. In Nov. 824 he promulgated a statute which reserved the supreme power to the secular poten tate, and he afterwards issued ordinances for the good govern ment of Italy. On his return to his father's court his step-mother Judith persuaded him to secure a kingdom for her son Charles, a scheme which was carried out in 829. 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.
When Louis was dying in 84o, he sent the imperial insignia to Lothair, who, disregarding the various partitions, claimed the whole of the empire. Negotiations with his brother Louis and his half-brother Charles, both of whom armed to resist this claim, were followed by an alliance of the younger brothers against Lothair. A decisive battle was fought at Fontenoy on June 25, 841, when, in spite of his personal gallantry, Lothair was defeated and fled to Aix. With fresh troops he entered upon a war of plun der, but he was compelled to surrender Aix to his brothers. In June 842 the brothers met on an island in the Saone, and agreed to an arrangement which eventually developed into the treaty of Verdun (843) By this Lothair received Italy and the imperial title, together with a stretch of land between the North and Mediterranean seas lying along the valleys of the Rhine and the Rhone. He aban doned Italy to his eldest son, Louis, and remained in his new king dom, engaged in alternate quarrels and reconciliations with his brothers, and in efforts to defend his lands from the attacks of the Normans and the Saracens. In 855 he fell ill, and divided his lands between his three sons. On Sept. 23 he entered the monastery of Priim, where he died six days later.