What Was The Roman Empire?

The Roman Empire was the successor to the Roman Republic and it saw autocratic rule concentrated in the hands of one man - the Emperor.  The first Roman Emperor was Augustus, who became sole ruler of what was then still the Republic in 31 BC though he was initially titled 'Consul' and became 'Emperor' later, as he reorganised the Republic into Empire.  

The Empire comprised all the territory under Roman rule.  Rome had been expanding beyond its city borders for many years before it was officially termed an Empire.  By 117 AD, the time of the Emperor Trajan, the Empire had reached its territorial peak, controlling around 2.3 million square miles of land across Europe, Asia and North Africa - one of the largest empires in history, stretching as far as Britain and Spain in the West and Mesopotamia (Iraq) in the East.  For hundreds of years it dominated Western Europe, culturally as well as via rule, and comprised most of its population.  It didn't, however, extend into the Germanic tribes of Northern Europe.

In 395 AD the Empire split in half.  The western half, controlled from Rome, came to an end in 476 when it fell to Germanic invaders.  The Eastern half, known as the Byzantine Empire and controlled from Constantinople, kept Roman law and culture alive for another millennium until it finally fell to the Turkish Ottoman Empire in 1453.