Muthanna's command of the Iraq front. After the transfer of Khalid bin Walid to the Syrian front, Muthanna became the commander of the Muslim forces in Iraq. Khalid had taken one half of the troops with him to Syria, and left the other half with Muthanna in Iraq. With the reduction in the strength of the troops in Iraq, Muthanna was not in a position to take the offensive. He accordingly withdrew from the advanced posts, and cantoned the troops at Hirah.

Shahr Iran. For long the affairs in Persia had been in a state of disarray because of succession disputes. With the accession of Shahr Iran, stability was restored to Persia. The new king was ambitious and on assuming authority, he decided to take action against the Muslims and drive them from the soil of Iraq. Khalid who was a terror for the Persians was no longer in Iraq. The Muslim forces on the Iraq front had been considerably reduced. The Persian Kisra, therefore, felt that it was the ideal time to take action against the Muslims. A large Persian force was mustered, and placed under the command of a veteran General Hormuz.

Letter to Muthanna. Shahr Iran sent an insulting letter to Muthanna demanding immediate withdrawal of the Muslim forces from Iraq. The Kisra observed that the Muslims were so despicable before him that he was not sending the main Persian army against them. He was sending an army of "fowl men and swine herdmen." Muthanna replied that the Muslim forces were not there to withdraw, they were there to fight and they would give a good account of themselves. Commenting on the Kisra's letter, Muthanna said that it appeared that he was either a braggart or a liar, and in any case unless he chose to see the light of reason, his army of "fowlmen and swine herdmen", God willing, would be destroyed.

Battle of Babylon. In spite of the heavy odds against him, Muthanna did not lose nerve. He decided to give the battle away from Hirah. He accordingly marched with his troops from Hirah, crossed the Euphrates, and arrived at the site of Babylon where the Persian forces under Hormuz were already camped. When the battle began, the Persians had all the advantages in their favor. The Persians had a fierce war elephant in front of their ranks, and the beast threw the Muslim ranks in confusion and paralyzed their action. At this stage, Muthanna directed his archers to aim every arrow at the beast. Soon the beast was pierced with innumerable wounds. It groaned, staggered and fell. With the fall of the beast, the offensive of the Persians lagged Muthanna ordered his men to fall at the Persians. In the hand to hand fight that followed the Persians were routed. Hormuz fell on the battlefield. With his death the Persian resistance was over, and the Persians retreated post haste leaving thousands of their soldiers on the battlefield.

Muthanna's visit to Madina. Muthanna felt that the battle of Babylon was not the end of the matter. The Persians had considerable resources at their disposal, and they were likely to raise a still larger army against the Muslims. Muthanna wrote to Abu Bakr for reinforcement. There was some delay in reply from Madina and Muthanna dashed to Madina to apprise the Caliph personally of the situation in Iraq. When Muthanna arrived in Madina, Abu Bakr lay on deathbed. He, however, saw Muthanna, and listened to his account attentively. Thereafter Abu Bakr summoned Umar, and directed him to command levy for Muthanna. He said: "If I die this day wait not till the evening; if I linger till night wait not till the morning. Let not sorrow for me divert you from the service of Allah". This direction was the last official act of Abu Bakr as the Caliph.