After Amr b. Al-Aas had left for Egypt with 4,000 soldiers only, Umar on second thought considered that it was idle to expect to conquer such a large country as Egypt with vast manpower and reources with a meagre force of 4,000. Umar accordingly wrote a letter to Amr b. Al-Aas asking him to come back. A post script was however added:
"If you receive this letter when you have already crossed into Egypt then you may proceed. Allah will help you and I will also send such reinforcement as may be needed."
The letter was sent through a special messenger Uqba bin Amr.
Uqba caught up Amr at Rafat a little short of the frontier. Guessing what might be in the letter, Amr ordered the army to quicken up its speed. Turning to Uqba, Amr said that he would receive the Caliph's letter from him when the army had halted after the day's journey. Uqba being not aware of the contents of the letter agreed and marched along with the army.
The Muslim army halted for the night at Shajratein. This was a place well within the Egyptian territory. Now the Caliph's letter was received and read. Amr consulted his companions as to the course of action to be adopted. The unanimous view was that as they had received the letter on the Egyptian soil, they had the permission to proceed. To the Caliph, Amr wrote:
"We have received your letter when we have reached Egypt. Therefore in the fulfilment of destiny we proceed seeking Allah's blessing."
Wben Umar received the reply, he decided to watch further developments.
From Shajratein, the Muslim army marched to Areesh. It was a small town where there was no garrison. No resistance was offered and the citizens offered allegiance on the usual terms. That was the Eid-uz-Zuha day. The Muslims celebrated the Eid festival at Areesh and offered the usual sacrifices.
In the later part of December 639 the Muslim army reached Farma. It was a fortified town manned by a Byzantine garrison. The Muslims besieged the town. There were sallies and counter sallies with no decisive result. The siege dragged on for two months. Towards the fall of February 640 an assault group led by Useifa b. Wala assaulted the fort and captured the gate through which the rest of the Muslim army entered. Thereupon the Byzantine resistance collapsed and the city was captured by the Muslims.
After the fall of Farma the Muslims marched to Bilbeis 40 miles from Memphis. It was a fortified town, and the Muslims besieged it. The siege lasted for a month, and towards the end of March 640 the city surrendered to the Muslims.
From Bilbeis the Muslims marched to Babylon. Amr had visualised that the conquest of Egypt would be a walk over. This expectation was belied. Even at the outposts of Farma and Bilbeis the Muslims had to meet stiff resistance. The siege of Farma had lasted for two months and that of Bilbeis for one month.
Babylon was a larger and more important city and here resistance on a larger scale was expected. Amr nevertheless persevered and pushed on to Babylon.