After the conquest of Khuzistan, the Muslims wanted peace. They wanted to leave rest of Persia to the Persians. Umar said:
"I wish there were a mountain of fire between us and the Persians, so that neither could they get to us, nor we to them."
But the Persians thought differently. The pride of the imperial Persians had been hurt by the conquest of their land by the Arabs. They could not acquiesce in the occupation of their lands by the Arabs.
After the battle of Jalaula the Persian emperor Yazdjurd went to Rayy and from there he moved to Khurasan where he set up his capital at Merv. >From Merv, the emperor sent a call to his people to rise to a man to drive away the Muslims from their lands. In response to the call, hardened veterans and young volunteers from all parts of Persia marched in large numbers to join the imperial standard.
The Persian forces were required to assemble at Nihawand south of Hamadan. Mardan Shah the son of Hurmuz was appointed to the chief command of the Persian forces. The banner of Kavah, regarded by the Persians as the harbinger of victory was unfurled and entrusted to Mardan Shah by the emperor.
As the Persian forces assembled at Nihawand their number exceeded 60,000. They were fully equiped, and were fired with the urge to drive away the Muslims from the Persian soil. As Mardan Shah surveyed the Persian forces, he thought that no power on earth could defeat them. He was very bitter against the Muslims. Addressing the Persian forces he said:
"Muhammad who brought the new religion to the Arabs, never troubled us. Then Abu Bakr became their ruler, and he too did not bother us except by taking plunder, and that only in the part of the Suwad which was adjacent to their land. Then after him came Umar, and his rule has become very long. He has taken the Suwad and Ahwaz and ridden rough shod over them. He is very ambitious and war-like, and it appears he will not rest content until he has conquered the whole of Persia and made you slaves. He is coming to you, if you do not go for him. He has already done us considerable damage. We cannot allow him any further liberty. We must take the initiative and drive the Muslims from our lands. We should recapture the entire territory that he has captured from us. I will not be satisfied till our forces drive away the aggressors to the desert and engage them in their own land."
There was great enthusiasm among the forces, and on sacred fire they took the oath to carry on the war against the Muslims to the bitter end.
The news of the Persian resolve to fight, and their preparations on a large scale were communicated by the Muslim scouts to Qubaz the Commander of the Muslim forward troops at the border post on Kirmanshah road. Qubaz reported the matter to Ammar bin Yasir, the Governor of Kufa. Ammar in turn addressed a letter to Umar and sought his instructions. The letter was sent through a special messenger.