The words ‘Tsar’ and Kaiser are derived from Caesar (100-44 BC). His adopted son Gaius Julius Caesar Octavianus (63 BC-14 AD) succeeded in permanently establishing the one-man rule or emperorship. However, he never called himself king, let alone emperor, but princeps inter pares. After 27 BC, he was called Augustus (the venerable), a title conferred by the senate.
Augustus and his successors transformed the Roman Republic into an empire; the republican constitution existed only in name. Constantine the Great (c. 272-337) established a new centre of power in Constantinople (or Byzantium, its ancient name) in 326.
Christianity became the formal religion of the emperors around 395. Christianity changed the empire, emperorship, society and art. Christian emperors regarded themselves as God’s deputy on earth, and profound changes became visible, though they drew on the ancient Roman heritage. Christian iconography was based on pagan Roman, Jewish, and oriental examples; the ecclesiastical organisation was a copy of the Roman administration, and Latin became the language of the Church (though Greek was an essential Christian language in the eastern part of the Empire, and the first scriptures were in Greek).
Following the end of the Roman Empire in the West in 474, the Eastern emperor laid claim to the Roman title. The East-Roman Empire evolved into the Byzantine Empire, which ended in 1453 with the Ottoman conquest.
The Greek-Byzantine Empire should have a long-lasting impact on later European Empires. The Greek alphabet and Greek orthodoxy were introduced in large parts of Eastern Europe, and Russian and Bulgarian Tsars copied Byzantine religion and culture.
In the second half of the 8th century, a new Frankish Empire emerged north of the Alps. Charlemagne, crowned emperor by Pope Leo III in Rome in 800, established a new European empire.
Charlemagne saw his reign as the continuation of ancient Roman emperorship. The Carolingian Empire ceased to exist in 924 and was succeeded by the Ottonian Empire in the East-Frankish-Saxon part of the Carolingian Empire.
The coronation of Otto the Great by Pope John XII in Rome confirmed the exclusive right of the Pope to bestow the imperial crown. It was the start of the medieval secular and religious political and investiture conflict and the centuries-long German (later Habsburg) involvement in Italian lands. The vision of the Roman Empire’s renewal persisted in the Holy Roman Empire until its dissolution in 1806.
Competitors were never far away, such was the prestige of Europe’s first emperors. Napoleon Bonaparte and Napoleon III copied the Roman heritage. One of the long-lasting Roman contributions is the Romanisation of the concept of emperorship.