Title of the sovereigns of the ancient Roman empire and, by derivation, various later European rulers, also applied to certain non-European monarchs. Caesar Augustus was the first Roman emperor. Byzantine emperors ruled at Constantinople until 1453. Charlemagne became the first of the Western emperors (later Holy Roman emperors) in 800. After Otto I became emperor in 962, only German kings held the title. In other parts of Europe, monarchs who ruled multiple kingdoms (e.g., Alfonso VI, who ruled Léon and Castile) sometimes took the title emperor. Napoleon's assumption of the title, as a putative successor of Charlemagne, was a direct threat to the Habsburg dynasty. Queen Victoria of Britain took the title empress of India. Non-European peoples whose rulers have been called emperor include the Chinese, Japanese, Mughals, Incas, and Aztecs.

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