Babylon was across the other bank of the Euphrates. It was a place of historic importance having been at one time the capital of Iraq. It was also a place of strategic importance and was the gateway of the Suwad, the land between the Euphrates and the Tigris.
About two years earlier the Muslims had fought a battle at Babylon under Muthanna when they had occupied Babylon. Later on the eve of the battle of Qadisiyya when the Muslims withdrew from all advance posts in Iraq, they abandoned Babylon Having won the battle of Qadisiyya, the Muslims now advanced to re-conquer Babylon.
Some time in the middle of December 636, the Muslims crossed the Euphrates and camped outside Babylon. There was a big concentration of the Persian forces at Babylon commanded by the veteran Generals, Feerzan, Hormuzan, Mihran and Nakheerzan.
There was a battle at Babylon. The details of the battle have not been recorded in any history. It appears that there was disunity among the Persians, and they could not put up a stiff resistance against the Muslim charge. Hormuzan with his forces withdrew to his home province Ahwaz. On his withdrawal, the other Generals also pulled back their forces and withdrew northward.
After the Persian forces had left, the citizens of Babylon formally surrendered. They were afforded protection under the usual terms. They volunteered to cooperate with the Muslims in their fight against the Persians. They furnished a good deal of useful information about the disposition of the Persian forces. Some Babylonians were employed in the construction of roads and bridges.