Before the battle of Nihawand the policy of Umar was that the Muslims should be content with what they had acquired in Iraq, and should leave Persia proper to the Persians and their emperor Yazdjurd. The battle of Nihawand showed that as long as Yazdjurd was there and the Persians smarted under the pain of the loss of their empire, the danger of Persian confrontation was always there. It now came to be felt that in order to ensure the security of the territories that the Muslims had wrested from Persia, it was necessary that the Persian power should be crushed once for all, so that no danger could come to the Muslims from that quarter in the future.
The psychological moment for striking a blow at Persia was immediately after the battle of Nihawand when as a consequence of the defeat the Persians were paralysed. Under the circumstances Umar agreed to change his policy with respect to Persia. Having adopted the forward policy, the problem before Umar was: where should the Muslims launch the next attack against Persia. There were three alternatives: firstly, an attack against Fars in the south; secondly an attack against Azarbeijan in the north; and thirdly an attack against Isfahan in the centre. Umar summoned Hormuzan and sought for his advice as to where the Muslims should launch the attack. Hormuzan said:
"Fars and Azarbeijan are two arms and Isfahan is the head. If you cut off one arm, the head and the other arm will be there. If you cut off the head, the arms will fall by themselves. So better start with the head."
This advice appealed to Umar, and he ordered that the first attack against Persia should be launched against Isfahan. Umar appointed Abdullah bin Abdullah bip Utban to the chief command of the force that was to launch the attack against Isfahan. Abdullah b. Warqah al Asadi was appointed to command the right wing, and the left wing was placed under the command of Asmata bin Abdullah. Abdullah bin Abdullah accordingly set off with an army from Iraq, and marching via Nihawand made straight for Isfahan.
In an outlying district town of Isfahan, the advance of the Muslim force was resisted by a Persian detachment. The Persian force was commanded by Shahr Baraz Jazwiah. He was a man advanced in age. He suggested that instead of a battle between the forces it would be enough if there was a personal duel between a champion from the Persian forces and a champion from the Muslim forces. The Muslim commander agreed to the proposal. Abdullah bin Warqah the right wing commander of the Muslim forces volunteered to fight the duel. Shahr Baraz Jazwiah and Abdullah bin Warqah fought the duel. Shahr Baraz was advanced in age while Abdullah was young. The Persian champion was more experienced and skilful. Ultimately the age factor prevailed and Abdullah bin Warqah succeeded in killing Shahr Baraz Jazwiah. There was a further fight and Astandar the ruler of the district surrendered. A peace pact was drawn "hereunder the Persians agreed to pay Jizya.
Thereafter the Muslim forces marched to Rayy which was a suburb of Isfahan. Here the advance of the Muslims was resisted by a Persian force. The Muslims launched the attack. After some fight the Persians retreated to Isfahan. The Muslims advanced and laid siege to the city of Isfahan.
Here the Muslims received further reinforcement. One large corps came under Ahnaf bin Qais, and another column from Basra came under the command of Abu Musa. The Muslims blocked all points of access to the city of Isfahan. The position of the besieged soon became precarious. At this juncture the Commander of the Persian forces, Fazusfan suggested that instead of involving so many persons in bloodshed it would be advisable if the issue was decided between the two commanders by personal duel, the winner taking all.
Abdullah the Commander of the Muslim forces accepted the proposal. The two generals met on horseback in the plain outside Isfahan to fight a duel. Abdullah enquired of Fazusfan whether he would like to strike first. The Persian General said that he would strike first The Persian General struck. As a result of his stroke the saddle on the horse of Abdullah broke. He slipped off the horse and landed on the ground. He immediately rose up to jump on the bare back of his horse.
Now it was the turn of Abdullah to strike, but before he could strike his adversary surrendered. The usual terms were offered and the Persian General agreed to pay Jizya. A peace pact was drawn accordingly.
From Isfahan, one Muslim contingent proceeded to Kashan and captured it. Another column proceeded to Qum and likewise captured it. Now Isfahan and the region around it was in the occupation of the Muslims. The Muslims had succeeded in severing the head of Persia and that was a great blow.
The Isfahan campaign came to a successful conclusion some time in the early months of 642 A.D.