Another revolution in Persia brought Yazdjurd to the throne of Persia. He was young and intelligent, and on coming to the throne his principal concern was to take effective steps to drive away the Arabs from the soil of Iraq.
Heretofore some battles had been fought on the soil of Iraq, but these had not been decisive. "The Muslims had occupied some areas, but their hold had not been firm. In the counter movements of the Persians the Muslims were pushed out of such areas. The Muslims retaliated and occupied such areas again. And again they abandoned them either of their own accord for strategical reasons or were pushed back. This to and fro process had been repeated several times, and this had led to political instability in the Suwad, the fertile area between the Euphrates and the Tigris.
Yazdjurd decided to organise things in a big way, and mobilise the resources of his empire for a titanic struggle with the Arabs. The Persians mustered a strong force under the veteran General Rustam. The force fully armed and equipped was cantoned at Sabat near al-Madain.
When these developments were reported to Umar, he realised that the scanty. Muslim forces in Iraq under the command of Muthanna were exposed to great danger. The Caliph ordered Muthanna to abandon Hira and other advanced posts in Iraq and to withdraw to the edge of the desert. Musanna pulled back his forces and stationed them at Sharaf close to the edge of the desert. In the southern sector the Muslims also pulled back and encamped in the hills of Ghuzayy.
The entire Suwad and all the main cities of Iraq were once again under Persian occupation. The war against the Persians, had to start once again from the periphery. Umar gave the call to Jihad. Throughout the Arabian peninsula messages were sent to the Governors and the chiefs of tribes to muster in full strength at Madina. The command of Umar was:
"Leave none who has weapons or a horse or strength or intelligence. Take him and send him to me. Hurry, O hurry!"
The response to the call was encouraging. Volunteers began to pour into Madina. Umar organised the camp at Sirar three miles from Madina on the route to Iraq. In March 636 A D. the first concentration of troops was complete, and Umar moved in person to the camp at Sirar leaving the administration at Madina to the charge of Ali.
Umar addressed the troops mustered at Sirar, apprised them of the situation in Iraq, and invited their reaction. The congregation said with one voice, "Go, and we go with you for the glory of Islam." Umar said, "Prepare for war, and I will go with you unless some better counsel comes forth."
Umar summoned a council of war at Sirar to which leading Companions were invited. The council was required to advise whether the campaign in Iraq shoutd be led personally by Umar, or should some one else be appointed to the command.
Ali said, "Go yourself for that will have a greater psychological effect both upon the Muslims as well as the enemy". Talha endorsed this view.
Abdur Rehman bin Auf said, "Stay, and send the army; and the will of Allah in respect of your wishes will be manifested in the fortunes of your army. If it is defeated, it will not be your defeat; but if you are killed or defeated, it would be a humiliation and a terrible blow to Muslim prestige."
After discussion, and the weighing of the pros and cons the consensus emerged in favour of the view advocated by Abdur Rahman bin Auf.
The Caliph next sought advice to the person who should be appointed as tbe Commander-in-Chief of the Muslim forces in Iraq Abdur Rahman bin Auf proposed the name of Saad bin Abi Waqas.
Saad bin Abi Waqas was at that time the Governor of Nejd. He was one of the earliest converts to Islam. He was among the 'Ashra Mubashara', the Ten Companions who had been given the news of Paradise in their life time. He was the only man to whom the Holy Prophet had said, "I sacrifice my father and mother to you." He was the maternal uncle of the Holy Prophet.
Umar said, "I know that Sand is a brave man He fought at Badr and Uhud. My only anxiety is that he does not have sufficient knowledge about the strategy of war."
Othman said, Saad should be appointed to the command, and he should be instructed to seek counsel from men of experience and knowledge of war, and not act without their advice." This view was endorsed by all and ultimately agreed to.
The following day Umar ordered a congregation of the army at Sirar, and addressed them as follows:
"Lo! Allah Most High and Mighty has gathered his people to Islam and his joined their hearts and made them brothers one to another. The Muslims are like one body of which the entire body suffers, if any part suffers. It is incumbent upon the Muslims to decide their affairs in a council of men of judgement. The troops must follow the one appointed to command by mutual agreement and consent; and the one appointed to command must accept the decision of men of judgement in the strategy of war. O people, I am just one of you, but men of judgement have dissuaded me from going with you. I have decided to remain here, and send another person in command; and I have consulted all in this matter."
Saad was called from Nejd, and as he appeared before Umar, the Caliph said:
"I have appointed you Commander of the war in Iraq. Remember my words for you are proceeding on a difficult and fearful mission in which right can only prevail."
In May 636 A.D., Saad bin Abi Waqas marched from the Sirar camp with an army of 4,000 men. At the time of departure Umar prayed for the success of the mission they had undertaken. His parting instruction to Saad was:
"Stop when you get to Zarud and disperse in the region. Urge the people there to join you ami take all who have courage, intelligence, strength and weapons."
Umar promised that he would send more and more of help. He said that he would hurl every chief, every noble, and every warrior in Arabia against the Persians.
As the army under the command of Saad marched past Umar, the Caliph raised his hands in prayers and said:
"O Mighty Allah! These people are going to fight in your way. Bless them with victory."