Despite the initial reservations of his advisers, Abu Bakr recognised the military and political prowess in Umar and desired him to succeed as caliph. The decision was enshrined in his will, and on the death of Abu Bakr in 634, Umar was confirmed in office. The new caliph continued the war of conquests begun by his predecessor, pushing further into the Sassanian Empire, north into Byzantine territory, and went into Egypt. These were regions of great wealth controlled by powerful states, but the long conflict between Byzantines and Persians had left both sides militarily exhausted, and the Islamic armies easily prevailed against them. By 640, they had brought all of Mesopotamia, Syria and Palestine under the control of the Rashidun Caliphate; Egypt was conquered by 642, and the entire Sassanian Empire by 643.