In this matter, victory or defeat is not dependent on the smallness or greatness of forces. It is Allāh’s religion which He has raised above all faiths, and His army which He has mobilised and extended, till it has reached the point where it stands now, and has arrived its present positions. We hold a promise from Allāh, and He will fulfil His promise and support His army.
The position of the head of government is that of the thread for beads, as it connects them and keeps them together. If the thread is broken, they will disperse and be lost, and will never come together again. The Arabs today, even though small in number are big because of Islam and strong because of unity. You should remain like the axis for them, and rotate the mill (of government) with (the help of) the Arabs, and be their root. Avoid battle, because if you leave this place the Arabs will attack you from all sides and directions till the unguarded places left behind by you will become more important than those before you.
If the Persians see you tomorrow they will say, “He is the root (chief) of Arabia. If we do away with him we will be in peace.” In this way this will heighten their eagerness against you and their keenness to aim at you. You say that they have set out to fight against the Muslims. Well, Allāh detests their setting out more than you do, and He is more capable of preventing what He detests. As regards your idea about their (large) number, in the past we did not fight on the strength of large numbers but we fought on the basis of Allāh’s support and assistance.
 When some people advised Caliph ‘Umar to partake in the battle of al-Qādisiyyah or Nahāwand, he finding it against his personal inclination, thought it necessary to consult Amīr al-mu’minīn, so that if he advised against it he would plead before others that he had stayed back on Amīr al-mu’minīn’s advice, but also if he advised partaking in the battle some other excuse would be found. However, unlike others, Amīr al-mu’minīn advised him to stay back. The other people had advised him to join in fighting, because the Holy Prophet did not send only others to fight but took part in it himself as well, keeping his close relations also with him. What Amīr al-mu’minīn had in view was that ‘Umar’s presence in the battle could not be beneficial to Islam, but rather his staying back would save the Muslims from dispersion.
Amīr al-mu’minīn’s view that “the position of the head of government is that of the axis around which the system of the government rotates” is a point of principle and does not concern any particular personality. Whether the ruler is a Muslim or an unbeliever, just or despotic, virtuous or vicious, for the administration of the state his presence is a necessity, as Amīr al-mu’minīn has explained elsewhere at greater length :
The fact is that there is no escape for men from a ruler good or bad. Faithful persons perform (good) acts in his rule while the unfaithful enjoys (worldly) benefits in it. During the rule, Allāh will carry everything to its end. Through the ruler tax is collected, the enemy is fought, roads are protected and the right of the weak is taken from the strong till the virtuous enjoy peace and are allowed protection from (the oppression of) the wicked. (Sermon 40)
The words which Amīr al-mu’minīn uttered in his advice are not indicative of any quality of Caliph ‘Umar except his being the ruler. There is no doubt that he held worldly authority, irrespective of the question of whether it was secured in the right way or wrong way. And where there is authority there is centering of people’s affairs. That is why Amīr al-mu’minīn said that if ‘Umar would go out the Arabs would follow him in large numbers towards the battlefield, because when the ruler is on the march the people will not like to stay behind. The result of their going would be that city after city would become vacant, while the enemy will infer from their reaching the battlefield that the Islamic cities are lying vacant, and that if these people were repulsed no assistance would reach the Muslims from the center. Again, if the ruler were killed the army would disperse automatically, because the ruler is as its foundation. When the foundation is shaken the walls cannot remain standing. The word “aṣlu’l-‘Arab” (the root chief) of Arabia has not been used by Amīr al-mu’minīn as his own but he has taken it from the Persians. Obviously in his capacity as the head of the State, Caliph ‘Umar was, in their view, the chief of Arabia. Besides, the reference is to the country, not to Islam or Muslims, so that there is no suggestion of any importance for him from the Islamic point of view.
When Amīr al-mu’minīn pointed out to Caliph ‘Umar that on his reaching there the Persians would aim at him, and that if he fell into their hands they would not spare him without killing, although such words would have touched the brave to the quick and would have heightened their spirits, ‘Umar liked the advice to stay back and thought it better to keep himself away from the flames of battle. If this advice had not been in accord with his personal inclination he would not have received it so heartily and would have tried to argue that the administration of the country could be maintained by leaving a deputy. Again when other people had already advised him to go out, what was the need for consulting Amīr al-mu’minīn except to get an excuse to stay back.