During the reign of Umar, the second caliph of the Arabs, Arab armies under the leadership of Amr ibn al-As invaded and conquered Egypt in 639 AD. Egypt was then a province of the Byzantine empire, ruled by a governor residing in Alexandria, the capital city. The inhabitants of Egypt, who were Monophysite Christians known as Copts, differed from the Melkite Christian Byzantines, who regarded monophysitical sects as heretical and treated them accordingly. The difference stemmed from disagreement over the nature of Christ. The Copts believed in his divine nature, while the Byzantines believed he was both human and divine. In consequence the Egyptians suffered from religious discrimination and persecution at the hands of their rulers, in addition to having to put up with a heavy burden of taxation to defray the expenses incurred through constant warfare between the Byzantines and their major rivals, the Sassanian empire. In brief, the population of Egypt resented the Byzantine domination of their country, and the burden of heresy that was laid upon them; they also resented the heavy taxation imposed upon them. Alienation of the population from their rulers was the hallmark of that period, as it was to be during successive periods due to differences in language, religion or ethnicity between rulers and ruled. Such alienation may not have mattered much to the population when government was efficient and administration just, but it was to become more important during periods of misrule and exploitation.