In the 6th century the area that had once been under the domination of the Kingdom of Kush converted to Christianity. It included the kingdoms of Alodia, Makuria and Nobatia, which rested on Egypt's southern border. Over a century later, the religion of Islam united the nomadic Arab tribes into an expanding military and political force by 632. In 640, the military leader 'Amr ibn al-'As conquered Egypt from the Byzantine Empire. To consolidate Muslim control over Egypt, it was inevitable to secure its western and southern borders. Amr accordingly sent expeditions to Byzantine North Africa and Makuria's Nubia.