The first two centuries of the Empire saw a period of unprecedented stability and prosperity known as the Pax Romana ("Roman Peace"). Rome reached its greatest territorial expanse during the reign of Trajan (AD 98–117). A period of increasing trouble and decline began with the reign of Commodus (177–192). In the 3rd century the Empire underwent a crisis that threatened its existence, as the Gallic Empire and Palmyrene Empire broke away from the Roman state, and a series of short-lived emperors, often from the legions, led the empire. The empire was reunified under Aurelian (). In an effort to stabilize it, Diocletian set up two different imperial courts in the Greek East and Latin West in 286. Christians rose to positions of power in the 4th century following the Edict of Milan of 313. Shortly after, the Migration Period, involving large invasions by Germanic peoples and by the Huns of Attila, led to the decline of the Western Roman Empire. With the fall of Ravenna to the Germanic Herulians and the deposition of Romulus Augustus in AD 476 by Odoacer, the Western Roman Empire finally collapsed; the Eastern Roman Emperor Zeno formally abolished it in AD 480. On the other hand, the Eastern Roman Empire survived for another millennium, until Constantinople fell to the Ottoman Turks of Sultan Mehmed II in 1453.