The exact history of the world is never outlined, and for a good reason. It's no use to get hung up in specifics of alternate history. Only the general historical movements are outlined, while the rest of history, somehow, happens the way it does in our timeline.
The first empire was that of Augustus, and Constantine, the Roman Empire we think of when we think of Rome. It is often called the Italian Empire, as in, "the time the Empire was ruled by Italians", and not as in "The Empire of Italy"
The second empire is that of Justinian, and Maurice, AKA the Byzantine Empire, or Greek Empire. It is only considered a separate empire from the first in hindsight, and until the rule of Phillip the great, was seen as simply a continuation of the first empire.
The third empire was only ever declared in hindsight, hundreds of years after the fact. It is the empire of Charlemagne, and Lothair, and is often known as the French Empire.
The fourth empire is that of Charles, and Phillip the great. Declared retroactively by Phillip, after inheriting both Austria and Spain from his father Charles, this empire was united when Germany, Italy, and Spain joined with England, which Phillip had claims to through his marriage, and invaded France, annexing the country, in 1560. This empire would truly last for only 20 years, starting to break apart with the dutch revolt, and finally being brought to a legal end at the end of the 30 years war, with France, England, Spain, and Austria being split into legally distinct kingdoms once more. This is often known as the German Empire.
The fifth and current empire was created in 1949, after WW2. After the United States decided not to get involved in the war, the UK and USSR were able to win the war, with the USSR occupying most of Europe. The USSR, now controlling all of europe to the Pyrenees, attempted to put communist governments in power in the various nations, and using nationalism to keep them seperate from one another, to prevent them working together against Moscow. This would prove a failure, as Russia was drastically overstretched, and a series of revolts from 1947-1949, explicitly opposing Russian attempts to appeal to Nationalism and impose Communism, ended with the creation of the modern Roman Empire. As most of the revolts had been funded by, and heavily organized by the United Kingdom, many call the current empire the British Empire; that is "the time the Empire was ruled by the British", and not as in "The Empire of Britain"
