The Muslim Caliphate continued to expand until by the turn of the 8th-century, it stretched from the Atlantic Ocean in the west to Central Asia in the east.

Under the first Caliph, Abu Bakr, force was used to prevent unrest and rebellion from causing the collapse of the new Islamic state, and the first raids were carried out into the territory of the Sassanid Empire.

It is unknown whether Umar intended from the outset to conquer both the Sassanid and the Byzantine Empire, or simply allowed raids, and then, perceiving their weakness, followed up with full-scale invasion.

[2][page needed] 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.

[3] But having captured Bilbeis, the Arabs moved again, eerily echoing Heraclius' successful campaign against the Sassanids just a short decade ago.

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.

Whether through the foolishness of the Byzantine generals, including Theodore, contributed to what then occurred, certainly Amr fought a brilliant battle at Heliopolis.

When Domentianus, governor of Faiyum heard of the defeat, he left his camp at Abûît and fled to Nikiû without even informing the locals that he was abandoning them to the Muslims.

[9] 'Amr then directed his troops to Delas, where he forced Apa Cyrus, pagarch of Heracleopolis Magna, to supply him and his army with boats to travel along the Nile.

John of Nikiû continues "Not content with that, 'Amr had the Roman magistrates arrested and their hands and feet fettered with chains or logs of wood: he extorted great sums of money, laid a double tax on the peasants, whom he forced to bring forage for his horses, and he committed innumerable acts of violence.

The next year and a half were spent on more maneuvers, skirmishes, and sieges before the formal surrender of the capital, Alexandria, took place on 4 November 641.

The loss of Egypt and Syria, followed later by the conquest of the Exarchate of Africa also meant that the Mediterranean, long a "Roman lake", was now contested between two powers: the Muslim Caliphate and the Byzantines.