another wholesale conquest. The increasingly frail Emperor was forced to depend on his generals, and the result was complete disaster. In 639, less than a year after the complete fall of the Sassanid Empire, an army of some 4,000 commanded by Amr ibn al-A'as, under orders of Omar, began the invasion of the Diocese of Egypt. That relatively tiny force marched from Syria through El-Arish, easily took Farama, and from there proceeded to Bilbeis, where they were delayed for a month. But having captured Bilbeis, the Arabs moved again, eerily echoing Heraclius' successful campaign against the Sassanids just a short decade ago. A small force, commanded by a charismatic and tactically brilliant commander went behind enemy lines, and caused chaos all out of proportion to their size. They laid siege to the fortress of Babylon near modern Cairo, which withstood Amr's siege for seven months. In the meantime Amr and his army marched (or rode) to a point on the Nile called Umm Dunein. The siege of this city caused Amr and his horsemen considerable difficulty as they lacked siege engines and overwhelming numbers. After finally taking Umm Dunein, Amr crossed the Nile to Faiyum. There, on 6 June 640, a second army dispatched by Omar arrived at Heliopolis (the modern Ain Shams) and began to lay siege to it. Amr retraced his route across the Nile River, and united his forces with those of the second army. They began to prepare for movement towards Alexandria – but scouts reported the approach of the Byzantine army. The battle At that point the united Arab army was confronted by a Roman army, which Amr, who had taken overall command, defeated at the Battle of Heliopolis. Just as Byzantine generals had failed in Syria, they did in Egypt, and the Empire's economically most valuable province after Anatolia was lost. The battle took place sometime in early to mid July 640, near the ancient city of Heliopolis, with Arab forces totaling approximately 15,000 under the command of Amr ibn al-A'as, and the Byzantine forces estimated at well over 20,000 under Theodore, commander of all Byzantine forces in Egypt. The Byzantine army could have responded sooner, but had not, for reasons that will never be known. Though historians such as Butler blame the treachery of the Coptic Christians as well as the failure of the Byzantine generals for the swift fall of the Exarchate