During the caliphate of Abu Bakr under the command of Khalid bin Walid the Muslims conquered a greater part of Iraq. In June 634 A.D., Khalid was asked to proceed to Syria, and Muthanna was left in command of the Muslim forces in Iraq.
With the departure of Khalid to Syria there was a lull in fighting on the Iraq front. Roughly the position was that the Persians held the territory to the east of the Tigris while the Muslims held the territory to the west of the Euphrates. The position about the territory between the two rivers known as the "Suwad" was somewhat obscure. It was no man's land. Sometimes parts thereof were occupied by the Persians and sometime by the Muslims. The people of the region thus kept shifting their loyalties, sometimes to the Persians and sometimes to the Muslims.
In July 634 a battle was fought between the Persians and the Muslims in the 'Suwad', somewhere near ancient Babylon. The Persians were under the impression that with the departure of Khalid and a diminution in the strength of the Muslim forces, it would be easy for them to defeat the Muslims. The battle of Babylon belied these hopes. Muthanna rose to the occasion, and after a violent battle the Persians were defeated.
Soon after there was a revolution in Persia. The Persian king was killed, and a lady Puran Dukht ascended the throne of Persia. The veteran General Rustam became the Commander-in-Chief of the Persian forces and he undertook to drive away the uncouth Arabs from the fertile land of Iraq.
Anticipating a Persian offensive on a larger scale under the new set up Muthanna felt that the Muslims should get ready for such a war, and for that more reinforcements were needed. In the third week of August 634 Muthanna went personally to Madina to get reinforcements for the Iraq front.
When Muthanna reached Madina, the Caliph Abu Bakr lay on the death bed. Muthanna waited on the dying Caliph, and apprised him of the situation in Iraq. He stated that the Persians were going to launch a big offensive, and that the Muslim forces in Iraq were too inadequate to meet the challenge. He made a strong plea for further reinforcements.
Abu Bakr though dying listened to Muthanna very carefully. He then sent for Umar the Caliph designate and when he came addressed him thus:
"Listen O Umar to what I say to you and act upon my words. I hope to die this very day and when I am dead let not the evening come upon you before you have exhorted the people to go with Muthanna. And if I survive till nightfall, let not the morning come before you have exhorted the people to go with Muthanna."
Ahu Bakr died that night, the 21st of August 634. He was buried the same night. After the funeral prayers, Umar exhorted the assembled Muslims to join Muthanna in the Iraq campaign.
On the morning of 22nd August the Muslims assembled to take the oath of allegiance to the new Caliph. After the ceremony was over Umar once again exhorted the Muslims to volunteer themselves for war on the Iraq front. Again there was no response. The Muslims were ready to join war in Syria but they hesitated in participating in a campaign against the Persians in Iraq. Although the Persians had been defeated in some campaigns, they were still held in awe, and the Muslims felt that the Persians were a hard nut to crack.
In his heart of hearts, Umar felt much upset at this want of response from the Muslims. He decided that whosoever was the first to offer his services for fighting on the Iraq front would be made the Commander-in-Chief of the Muslim forces in Iraq.
On the 23rd August, the Muslims were once again exhorted to Jihad in Iraq. Seeing the hesitation of the people, Muthanna took up the stage, and spoke eloquently of the need of pushing the war in Iraq to a successful conclusion. He said:
"Ye Muslims, listen to me! You should have no fear of the Persians. I have tested the courage of the fireworshippers and discovered that they are not at home on the battle-field. Years of luxury have made them easy going, and it would not be difficult for us to overpower them. We have already conquered most of the important districts of Iraq, and humiliated the Persians. With a little more effort and with the help of God we can become the masters of the whole of Iraq. It is incumbent on us to take the message of Allah and His Messenger to these fire worshippers and offer them the true faith of Islam."
Then Umar delivered a thrilling speech highlighting the mission of Islam. That appeared to move the audience. Then the Caliph asked for volunteers. Abu Ubaid the chief of the clan of Thaqafi rose up to offer his name. Umar welcomed the offer and said, "Abu Ubaid, I appoint you as the Commander-in-Chief of the Muslim forces in Iraq.
Then other people offered their names. By 25th August over 1,000 Muslims were ready to proceed to the Iraq front. Thereupon Muthanna left for Iraq. Abu Ubaid and his contingent were to follow.
Abu Ubaid was a man of great courage and dash, but he had no experience of actual fighting in any war. Umar was advised that for such an important campaign some veteran companion of the Holy Prophet seasoned in war should be appointed to lead the campaign.
"The Companions are entitled to such precedence because of their courage and love for Jihad. Here I have been giving the call to Jihad ever since we buried Abu Bakr, and I have had no response from the companions. Now that a young man who is not a companion has given the dead, I am determined to appoint him as the Commander-in-Chief. The Companions have lost this precedence by their own fault, and they should serve under a man who has given a greater show of courage."
Umar however appointed a few Companions as the advisers of Abu Ubaid. Abu Ubaid was instructed by the Caliph that he should act on the advice of these advisers.
After a few days when the necessary preparations had been made Abu Ubaid left Madina with a force of one thousand fighting men. He was further instructed that as he proceeded to Iraq he should recruit more fighting men from the tribes on the way.