Lothair I (lōthâr´), 795–855, emperor of the West (840–55), son and successor of Louis I. In 817 his father crowned him coemperor. He was recrowned (823) at Rome by the pope and issued (824) a constitution, proclaiming his right to confirm papal elections. He twice (830, 833) revolted against his father, who favored Lothair's half-brother Charles (Charles the Bald, later Charles II) at his elder son's expense, and in 833, with his brothers Pepin and Louis the German, he succeeded in temporarily deposing Louis I. However, his brothers deserted him and restored Louis. Lothair retained only Italy. He later was reconciled with his father, who in 838 allotted him almost the whole eastern half of the empire, the west (France) going to Charles. After Louis's death Charles and Louis the German made war on their brother Lothair, who tried to reunite the whole empire under his sole rule. The battle of Fontenoy (841), although indecisive, checked Lothair. Renewing their alliance in 842 (see Strasbourg, Oath of), Charles and Louis the German forced (843) Lothair to sign the fateful Treaty of Verdun (see Verdun, Treaty of), which partitioned the empire of Charlemagne among the three brothers; Lothair retained the imperial title. He subdivided his domains among his sons Louis II, who was crowned emperor at Rome in 850, Lothair, king of Lotharingia, and Charles. In 855 he abdicated and became a monk.