One might say that the rise of the Roman Empire was caused principally by the failure of the Republic, but this is an ahistorical perspective, since there was no clear dividing line between the two. Augustus claimed to have restored the Republic and was not an emperor in the sense that we now understand the term. The Republic developed into an Empire slowly, first by conquest of the surrounding Italian city states, then by controlled expansion across Southern Europe, Asia Minor, and North Africa.

An efficient military machine which drove aggressive expansion was one factor which contributed to the rise of the Empire. Two others were economic growth, and a relatively tolerant foreign policy. These factors were linked, in that the principal demand Rome made of its newly-conquered people was that they pay their taxes. In matters such as religion and cultural practices, the Romans interfered very little.

These three factors in the rise of the Roman Empire also contributed to its fall. The military machine finally overreached itself, leading to a territory too large to be governed centrally, and therefore at the mercy of corrupt governors, many of whom had imperial ambitions themselves. The vast wealth of Rome and the luxurious lifestyles of its elite made it both a tempting and and a soft target for invaders such as the Huns and the Visigoths. Finally, the fact that Rome was so tolerant of other cultures and religions meant that there were no strong unifying bonds which extended across the Empire, so that most Roman subjects had no particular attachment to Rome.