The Roman Empire reached its peak—in terms of both territorial expansion and economic power—under the reign of Emperor Trajan, around 117 AD.... At that time, the empire's borders spanned nearly 6 million square kilometers, encompassing much of Europe, North Africa, and Middle East. The new provinces conquered by Trajan: Armenia, Assyria and Mesopotamia were already de facto lost by Romans back to Parthians. (5,957,000 km² of territory with a population of approximately 60,000,000 inhabitants)