The Roman Empire began as a monumental shift in power following centuries of expansion under the Roman Republic. In 27 BC, after a period of intense civil war and the assassination of Julius Caesar, his heir Augustus became the first emperor.
At its zenith in AD 117 under the rule of Emperor Trajan, the empire was a true transcontinental superpower, encircling the Mediterranean Sea, which the Romans called Mare Nostrum (Our Sea).
The eventual decline of the empire was a slow process driven by internal corruption, economic inflation, and constant pressure from Germanic tribes on the frontiers.
Territorial extent of the Roman Empire