In 27BC, the adopted son of Julius Caesar was recognised as “Imperator Augustus” (August Emperor), thus becoming overlord of the Western World.
In 330AD, his successor Constantine the Great removed the capital from Rome to Constantinople (New Rome). In 395, a permanent system of two joint emperors was instituted, the senior at Constantinople, the junior in Italy (first at Milan, then at Ravena). In 476, the last western emperor at Ravena was deposed and his regalia sent to Constantinople to formalise the end of the system of dyarchy. It was, however, reintroduced in 800 when Charlemagne was crowned Western Emperor at Rome. This time, however, the two emperors were not, as before, joint rulers of one empire, they were instead rulers of two separate states: the Holy Roman Empire in the West, and the (Eastern) Roman (Byzantine) Empire at Constantinople.
The Byzantine Empire was conquered by the Ottoman Sultan in 1453 whereupon he assumed the title of Emperor (Padishah). In 1472, however, Ivan the Great of Moscow married Sophie Palaeologue, niece of the last Byzantine Emperor and proclaimed himself the latter’s legitimate successor. There were thus thereafter two rival eastern empires: the Russian and Ottoman.
1n 1804, Napoleon I proclaimed himself Emperor of the French, as rival to the Holy Roman Emperor (who in 1804 took the title of hereditary Emperor of Austria). Napoleon was deposed and exiled in 1815 but his nephew, Napoleon III, revived the Empire of the French in 1852. He too was deposed, in 1870, whereupon the King of Prussia was proclaimed German Emperor thus perpetuating the system of two western Emperors. To obtain equal status, Queen Victoria was proclaimed Empress of India in 1877 , thereby adding a third western empire!
Two of the rival western empires and the two eastern ones all destroyed each other in the First World War. The Russian was the first to fall (in 1917) then the Austrian and German ones (in 1918) followed by the Ottoman in 1922. This left the King of England as the only European ruler with imperial status (as Emperor of India). In 1936, he was emulated by the King of Italy taking the title of Emperor of Ethiopia. These two King-Emperors were not destined to reign long. King Victor Emanuel III of Italy renounced his imperial title in 1936. King George VI did likewise in 1948.
For the first time for nearly two millennia, Europe was thereafter left without an emperor. Throughout that period rulers had tried to perpetuate the Roman ideal of a senior European Imperial Head of State. Its passing left a void. The Soviet Union (and its successor the Commonwealth of Independent States) then the Third Reich (and its successor the European Union) have tried to fill the void, without success.