After the conquest of Khuzistan, Umar wanted peace.;Though considerably weakened, the image of the Persian Empire as a fearsome superpower still resonated in the minds of the newly-ascendant Arabs, and Umar was wary of unnecessary military engagement with it, preferring to leave the rump of the Persian Empire alone.

After the defeat of the Persian forces at the Battle of Jalula in 637, Yazdgerd III went to Rey and from there moved to Merv, where he set up his capital and directed his chiefs to conduct continuous raids in Mesopotamia. Within four years, Yazdgerd III felt powerful enough to challenge the Muslims again for control of Mesopotamia. Accordingly, he recruited 100,000 hardened veterans and young volunteers from all parts of Persia, under the command of Mardan Shah, which marched to Nahavand for the last titanic struggle with the Caliphate.

The Battle of Nahavand was fought in 642 between Arab Muslims and Sassanid armies. The battle is known to Muslims as the "Victory of Victories." The Sassanid King Yazdegerd III escaped to the Merv area, but was unable to raise another substantial army. It was a victory for the Rashidun Caliphate and the Persians consequently lost the surrounding cities including Spahan (renamed Isfahan).