In the campaign against the Persians concentrated at Nihawand, Umar appointed No'man bin Muqarrin as the Commander-in-chief of the Muslim army.
Noman bin Muqarrin was the son of Ubaid bin Aus an Ansar of Madina. Ubaid participated in the battle of Badr, where he captured four infidels and tied them up in one chain. For this act of binding, the Holy Prophet gave him the name of Muqarrin, the Binder.
No'man had several brothers, and all of them were good Muslims and warriors. They played important roles in the apostasy wars under Abu Bakr. They fought under Khalid bin Walid in the wars in Iraq. Later Noman fought under Saad b. Abi Waqqas. After the battle of Kaskar, Noman was appointed as the Administrator of Kaskar district.
Noman was not happy with the civil appointment. He longed for active service. He wrote to Umar that Saad had appointed him to collect taxes, but he would personally prefer active service on some front to carry on the holy war.
When Umar appointed Noman as the General to command the Muslim forces against the Persians, he addressed him a letter in the following terms:
"In the name of Allah, the Beneficent, the Merciful. From the slave of Allah, Umar, Commander of the Faithful, to Noman bin Muqarrin. Peace be upon you! I render praise unto Allah, and verily apart from Him there is no other god.
I have come to know that a vast army of Persians has gathered at Nihawand to fight against the Muslims. I appoint you the Chief Commander for the campaign. When you get this letter, go by order of Allah and with the help and support of Allah, along with those of the Muslims who are with you march against the enemy. You should halt at some well watered place near Nihawand. I have written to the people of Kufa to join you; and when your army is all together, advance to Nihawand.
In case you fall in battle, the commander of the army will be Hudheifa bin Al Yaman: if the falls, then Jareer bin Abdullah; if he falls, Mugheera bin Shuba; and if he falls, Ath'ath bin Qais.
Do not ill-treat your men or be harsh with them, for then you will come to harm. And do not withhold from them their rights, for then you will be unjust. And do not put them in a position of loss, because every single one of the Muslims is dearer to me than a lac diners.
Seek the help of Allah and repeat often 'There is no power to change and no strength except with Allah'.
And again peace be upon you."
Simultaneously Umar addressed letters to other Commanders who were to participate in the campaign under the chief command of No'man bin Muqarrin. Hudheifa bin Al Yaman was required to march with the bulk of the forces from Kufa. Abu Musa was to proceed with one third of the Muslim army stationed at Basra. A fresh force to be raised at Madina including many Companions and Umar's son Abdullah was to march to the front under the command of Mugheera bin Shu'ba.
Umar also organised an irregular force and this force was required to operate as raiders across the hills to disrupt Persian communications from Fars and Isfahan to Nihawand.