List of Roman emperors

The title of Augustus was conferred on his successors to the imperial position, and emperors gradually grew more monarchical and authoritarian.

[2] The style of government instituted by Augustus is called the Principate and continued until the late third or early fourth century.

[5] The territory under command of the emperor had developed under the period of the Roman Republic as it invaded and occupied much of Europe and portions of North Africa and the Middle East.

[8] After the Crisis of the Third Century, Diocletian increased the authority of the emperor and adopted the title dominus noster (our lord).

[b] Under Justinian I, in the sixth century, a large portion of the western empire was retaken, including Italy, Africa, and part of Spain.

[c] The line of emperors continued until the death of Constantine XI Palaiologos at the fall of Constantinople in 1453, when the remaining territories were conquered by the Ottoman Turks led by Sultan Mehmed II.

By the end of the third century, Rome's importance was mainly ideological, with several emperors and usurpers even beginning to place their court in other cities in the empire, closer to the imperial frontier.

In the Historia Augusta, an ancient Roman collection of imperial biographies, the usurper Pescennius Niger (193–194) is expressly noted to only be a tyrant because he was defeated by Septimius Severus (r. 193–211).

[37] This is also followed in modern historiography, where, in the absence of constitutional criteria separating them, the main factor that distinguishes usurpers from legitimate Roman emperors is their degree of success.

Likewise, Western Europeans didn't recognize the legitimacy of the Byzantine emperors and called them the 'emperor of the Greeks' or the 'emperor of Constantinople'.