had visualized that the conquest of Egypt would be a walkover. This expectation turned out to be wrong. Even at the outposts of Pelusium and Bilbeis, the Muslims had met stiff resistance. The siege of Pelusium had lasted for two months and that of Bilbeis for one month. Both battles were preludes to the siege of Babylon, which was a larger and more important city. Here, resistance on a larger scale was expected. After the fall of Bilbeis, the Muslims advanced to Babylon, near modern Cairo. The Muslims arrived at Babylon some time in May 640 AD. Babylon was a fortified city, and the Romans had prepared it for a siege. Outside the city, a ditch had been dug, and a large force was positioned in the area between the ditch and the city walls. The Muslims besieged the fort of Babylon some time in May 640. The fort was a massive structure 18 metres high with walls more than 2 metres thick and studded with numerous towers and bastions. A Muslim force of some 4,000 men unsuccessfully attacked the Roman positions. Early Muslim sources place the strength of the Byzantine force in Babylon about six times the strength of the Muslim force. For the next two months, fighting remained inconclusive, with the Byzantines having the upper hand by repulsing every Muslim assault. Some time in May 640 AD, 'Amr sent a detachment to raid the city of Fayoum. The Byzantines had anticipated this and had therefore strongly guarded the roads leading to the city. They had also fortified their garrison in the nearby town of Lahun. When the Muslim Arabs realized that Fayoum was too strong for them to invade, they headed towards the Western Desert, where they looted all the cattle and animals they could. They subsequently headed to Oxyrhynchus, which was defeated. The Arabs then returned to Lower Egyptdown the RiverNile.
Reinforcements from Madinah
In July, 'Amr wrote to 'Umar requesting reinforcement; but before the letter reached him, the caliph had already dispatched the first reinforcement, which was 4,000 strong. The army was composed mostly of the veterans of the Syrian campaigns. Even with these reinforcements, 'Amr was unsuccessful. By August 640, 'Umar had assembled another 4,000 strong force, which consisted of four columns, each of 1,000 elite men. Zubair ibn al-Awam, a renowned warrior and commander, veteran of the Battle of Yarmouk and once a part of Khalid ibn Walid's elite mobile guard, was appointed the supreme commander of army—'Umar had indeed offered Zubair the chief command and governorship of Egypt, but Zubair had declined. The column commanders included Miqdad ibn al-Aswad, Ubaidah ibn as-Samit, and Kharijah ibn Huzaifah. These reinforcements arrived at Babylon sometime in September 640. The total strength of the Muslim force now rose to 12,000, quite a modest strength to resume the offensive.
Fifteen kilometres from Babylon was Heliopolis. The Muslim army reached Heliopolis in July 640. It was the city of the Sun Temple of the Pharaohs and was famous for its grandiose monuments and learning institutions. There was the danger that forces from Heliopolis could attack the Muslims from the flank while they were engaged with the Roman army at Babylon. With some detachments, 'Amr and Zubair marched to Heliopolis. There was a cavalry clash near the current neighbourhood of Abbaseya. The engagement was not decisive, although it resulted in the occupation of the fortress located between the current neighborhoods of Abdyn and Azbakeya. The defeated Byzantine soldiers retreated to either the Babylon Fortress or the fortress of Nikiû. At an unguarded point of the wall of Heliopolis, Zubair and some of his picked soldiers scaled the wall of the city, and after overpowering the guards, opened the gates for the main Muslim army to enter the city. Heliopolis was thus captured by the Muslims. 'Amr and Zubair returned to Babylon.
Conquering of Babylon Fortress
When news of the Muslims' victory at Heliopolis reached Fayoum, its Byzantine garrison under the command of Domentianus evacuated the city during the night and fled to Abuit. From Abuit, they fled down the Nile to Nikiu without informing the people of Fayoum and Abuit that they were abandoning their cities to the enemy. When news of this reached 'Amr, he ordered a body of his troops to cross the Nile and invade Fayoum and Abuit. The Muslim soldiers captured the entire province of Fayoum without any resistance from the Byzantines. The Byzantine garrison at Babylon had grown bolder than ever before and had begun to sally forth across the ditch, though with little success. There had been a stalemate between the Muslim and Byzantine forces at Babylon, until the Muslim commanders devised an ingenious strategy and inflicted heavy casualties on the Byzantine forces by encircling them from three sides during one of their sallies. The Byzantines were able to retreat back to the fort, but were left too weak for any further offensive action. This situation forced the Byzantines to negotiate with the Muslims. The Byzantine general Theodore shifted his headquarters to the Isle of Rauda, whilst Cyrus of Alexandria, popularly known as Muqawqis in Muslim history, entered into negotiations with the Muslims, which failed to give any productive results. Emissaries were also exchanged between Theodore and 'Amr, leading to 'Amr meeting Theodore in person. After fruitless negotiations, the Muslims acted on 20 December, when, in a night assault, a company of hand picked warriors led by Zubair managed to scale the wall, kill the guards and open the gates for the Muslim army to enter. The city of Babylon was captured by the Muslims on 21 December 640, using tactics similar to those used by Khalid ibn Walid at Damascus. However Theodore and his army managed to slip away to the island of Rauda during the night.
Aftermath
The Byzantine commanders knew that the next target of the Muslims would be Alexandria. They accordingly prepared for the expected siege of the city. Their strategy was to keep the Muslims away from Alexandria by destroying their power through continued sallies and attacks from the fort. Even if this did not keep them away, it would weaken them morally and physically. It would be more of a war of patience than strength. In February 641, 'Amr set off for Alexandria from Babylon with his army. All along the road from Babylon to Alexandria, the Byzantines had left regiments to delay, and if possible, inflict losses on the advancing Muslims.
