The first era of Byzantine Empire lasted from about 324 to 640 AD. During this time, the separate identity of the empire was established. The first great period of the Byzantines occurred during the reign of Justinian I, who took the throne in 527. Justinian reconquered much of the territory that had fallen into barbarian hands.
The Byzantine Empire was ruled by Justinian I at it's height. His goal was to restore imperial rule, and with this he recovered Belisarius, Narsus, North Africa, Sicily, Italy, and Southern Spain. His efforts greatly expanded his empire, but left it stretched and bankrupt. Militarily, the Byzantine Empire was at its height in the first decades of the 11th century A.D., during the reign of the emperor Basil II.
As Justinian tried to expand the Byzantine Empire, efforts were unsuccessful. He faced challenges such as severe economic problems preventing the empire from developing a firm economic foundation. The empire also faced some high threats in the form of the Ottoman Turks and, the Christians of Western Europe who actually succeeded in capturing Constantinople for a time.