After two years three months and a few days since the oath of allegiance taken at Saqifa Abu Bakr breathed his last and as ordered by him Umar assumed the Caliphate.
The views of the distinguished companions and the intellectuals of Madina are not available with us. However, none of the dignitaries of those days contended or rose against Umar.
The day on which Umar was persuading Ali to take the oath of allegiance to Abu Bakr, Ali had said to Umar inter alia: "Draw the milk for him and take half of it for yourself. Strengthen his power today so that tomorrow the same is entrusted to you".
It has also been said that while Abu Bakr was lying in his death bed Abdur Rahman bin 'Awf spoke to him on the same lines regarding transfer of authority to Umar.
Furthermore, among the group of the Muhajirs and the Ansar who came to enquire about Abu Bakr's health there were persons who expressed their views regarding the appointment of Umar more clearly as compared with Abdur Rahman, and Talha was the foremost among them. On hearing the news of Umar's appointment he went to Abu Bakr during his illness (which culminated in his death) and said to him: "We have learnt that you have appointed Umar to succeed you as Caliph and given him authority over us although you are well aware of his harshness, obstinacy and malice for us. When you meet Allah and He questions you about it what reply will you give?”
Another man said to Umar: "In the first year it was you who selected Abu Bakr for rulership and this year he has appointed you to rulership".
Anyhow Umar came out of Abu Bakr's house holding a sealed decree in his hand. He showed it to the people and demanded their submission to himself. In the meantime a witty person approached Umar and enquired from him about the contents of the decree.
Abu Hafs said in reply: "I don't know". Thereupon the witty person said openly in public: "You don't know but we know that in the first year you elevated him to rulership and in lieu o(it he has elevated you to rulership this year". The views of the companions of all other people in regard to this decree are not so important; what is important is that none of the companions objected to it nor did they resist it. Abu Bakr had gained an experience from the oath of allegiance taken in his case and the utilized that experience on this occasion.
The experience was that Abu Bakr considered it his duty to select a caliph before his death and place all the military forces under his control and the military should recognize him officially.
In his turn Umar also utilized the experience gained by him during the Caliphate of Abu Bakr with regard to the companions and did not permit them to go out of Madina lest they should instigate the people to oppose him, or be influenced by the people, and the people should encourage them to display resistance and stability.
He, therefore, kept all the companions in Madina and took care that all of them should be under his vigilance and be obedient to his views and thinking, and incidentally he should avail of their thinking, information and ideas and be victorious in the battles. It was to achieve this end that he provided them all the amenities of life so that they might be comfortably placed.
More important than all these matters was the attention paid by him to administration and discipline, as a consequence of which all the military and State matters were disposed of according to his wishes.
Another factor which contributed to the stability of the conditions was that he persuaded the people to take part in jihad and consequently the strength of his government expanded and large incomes came to his hands. And as he was the authorized representative and trustee of the public treasury he acted justly in the matter of distribution of property  and was extremely careful in the matter of trusteeship and department of justice. And similarly he appointed experts to various jobs. They managed the courts and adminisnative organizations very minutely. Statistics of workers and unemployed persons had been prepared for the state organizations and the same statistics were the basis for the preparation of public budget and rationing for the military men and others. Welfare of the society was ensured in all respects and everyone, whether a worker, an unemployed person or a disabled person received ample share from the public treasury.
To sum up: He had provided in his territories all means and factors necessary for stability and perfect peace, so that all were happy and satisfied. Those, who were competent to take part in the construction and organization of the society, participated in building up the social structure of Islam, and he silenced those, who planned sabotage, by satisfying their greed.
Accordingly all his efforts were directed towards the strengthening of the foundation of the Islamic Government in all the conquered territories, and at last Umar transferred these subjugated nations from their old world to the new world of Muhammad.
In the meantime Umar resorted to spreading knowledge in a school of his and remained sleepless for the security of the interest of the State, He kept the path of work and earning livelihood open for everyone, and took steps to ensure that the affairs of the State should at no time be faced with intervention or be at a standstill, In this manner he solved the difficulties, one after the other, quite easily.
The disciples of the school of Umar exactly followed in his footsteps, they worked among the people with wisdom and peace of mind and cooperated, in the path of goodness and security of public interests, with everyone who had good intentions for the affairs of the State.
Amongst this group of people including the disciples of this school there were persons who earned their livelihood with much difficulty and by undergoing great hard ships, and endeavoured day and night to solve the difficult problems. They arranged the spreading of Islamic learning with great sincerity and cooperated with one another for the strengthening of the Islamic Government on the basis of goodness and felicity.
Arnmar was one of the guided, trained, and refined persons of this group who had acquired perfect guidance and possessed an attractive spirit of leadership. Possibly it was due to his prolonged silence and quietness that he was considered to be happy and satisfied during the rule of Umar and possibly it was due to his antecedents that Umar felt lesser anxiety on his account as compared with others,
Incidentally it should be kept in mind that one of the policies of Umar was that he should keep the Hashimites and their followers, as well as those, who desired their rulership, away from the centres of government, so that they might not propagate in favour of Bani Hashim, He calculated that in that event (i.e, if they were allowed to support the cause of Bani Hashim) troubles and difficulties would arise, and it would not be possible to control them. 
Keeping all the pros and cons of the matter in view and the fear which he had in his mind on account of the prevailing conditions and the inclination of the people towards Bani Hashim, Umar felt that Ammar was neither a tyrant nor was he covetous or authority. Besides all this Ammar was a person who was not concerned with anything other than piety, moral soundness and correct thinking. It can however be safely said of him that he knew everything but he kept quiet. Of course, he was a staunch follower of Ali. His general condition did not cause any anxiety.
Now for certain reasons it became expedient that Ammar, too, should be selected for governorship. It was thought that on account of his belonging to the Hashimite party his appointments as governor might possibly become the cause of the satisfaction of those belonging to that party. And even if they were not satisfied with this appointment it would at least help the Caliph proclaim his impartiality, justice and innocence to the people. Although we do not know it for certain but it is possible that the Caliph thought that by appointing Ammar as the Governor of Kufa he would be able to attract him to himself and thus reduce the number of the supporters of Imam Ali.
However, we may suppose whatever we like, but what is certain is that if we do not say that Umar appointed Ammar to the governorship of the biggest province of the Islamic territories we can say that he appointed him as the governor of one of the biggest provinces viz. Kufa.
However, Umar sent Abdullah bin Mas'ud and Uthman bin Hunayf (who belonged to the Shi'a faith) along with Ammar as ministers, and ordered that Abdullah was to be responsible for financial and educational matters and Uthman was to take care of the suburbs of Kufa and the gardens. Ammar was to exercise supervision over these two persons.
Umar wrote a letter to the people of Kufa on these lines: "I am sending Ammar Yasir as your governor and the Ibn Mas'ud as the teacher and minister. I have made the Ibn Mas'ud the trustee and treasurer of your public treasury. These two persons are distinguished companions of the Holy Prophet Muhammad (s) and are included among those who fought at Badr. You should act according to their directions and obey their orders and follow them. Incidentally you should know and be aware that my sending Ibn Mas'ud to you is tantamount to my relinquishing the right of benefiting from him and giving you preference over myself. I have appointed Uthman bin Hunayf for the administration of the suburbs. I have fixed the boarding expenses and the ration of this body of men to be one sheep. Half the meat and the belly of the sheep shall be given to Ammar. As regards the remaining half one fourth shall go to Uthman bin Hunayf".
In any case the son of Sumayya or Ammar Yasir was the same strongwilled powerful person and the mighty rival who was not cowed down by the torture to which he had been subjected by Abu Jahl. He set a fine example of perseverance and steadfastness. Even now he was not one who should show pride, and in spite of the fact that he had attained a high position, and had become the chief of all dignitaries and chiefs, he was as humble and meek as ever. He had acquired these distinctions by his faith in truth and by the blessings of the religion which was given to him by Allah and explained by Prophet Muhammad (s). On becoming the chief such a person should behave like this and for this very reason his way of working and moral attitude was exactly like that of those days when he was just like an ordinary subject.
Governorship and authority was something which had no effect on him, because his personality was much above these things. The only use which he made of his position was that he put the power into action for the benefit of the society. He was a person who applied law to himself in the same way in which he applied it to others.
As soon as Ammar was free from public affairs he took off the official dress and once again became the same quiet, tranquil and reflective as before. He did not fly high and did not display superiority. He had no sentry and no screen or chamberlain. He lived among the people as an ordinary man or even below an ordinary man. Many a time people insulted him or contended with him but he did not display any anger. Sometimes he was charged for a commodity more than its real price, but he did not take advantage of his position and power as the governor. Sometimes others spoke rudely to him but he replied to them only with a smile.
When he spoke he did not say anything vain or absurd and if a person committed transgression against him he prayed and sought Divine mercy and prosperity for him.
Ibn Sa'd, the author of Tabaqat, relates the following story about Ammar: "One day Ammar purchased fodder worth one dirham from a man and asked him to give the string with which the fodder had been tied, but the seller declined to give the string. Ammar pulled up the string and cut it into two halves. Then he tied the fodder with the same string and, being the Governor of Kufa, he put it on his shoulder and set off for his place".
He goes on to say: "Ammar sent one of the members of the Attar and Tamimi tribe to the opening of Mah on a military expedition. Later it became necessary that a letter should be sent to him. He wrote the letter and took it personally to deliver it to his appointee. When that man saw that Ammar had come personally to deliver the letter he became annoyed because he knew that by bringing the letter in the battlefield Ammar would become entitled to share the possible booty. He received Ammar coldheartedly and said with extreme impudence: "You with a cropped ear! Have you come to share our booty?"
Ammar who was the Governor and Ruler of Iraq laughed on hearing these words and said: “You have insulted my best ear”. By the best ear Ammar meant the same ear which was chopped off in the Battle of Yamama. Anyhow the battle ended in victory, but the Tamimi man declined to allow Ammar to share the war booty. The matter was then referred to the Caliph (Umar) who decided that Ammar should get a share of the booty in accordance with the rule: "Whoever is present in the battlefield gets a share of the booty”!
With all these good manners and meekness Ammar was a prudent man with decisive character and at the same time possessed dignity and was obeyed and loved. None of the events which took place during the period of other rulers of Kufa occurred during his rule and his reputation remained always stainless. This was so, because he was a wise and pious person who was within the limits of rules and regulations and did not add anything to them.
Ibn Sa'd (the historian) has narrated that once a question was asked from Ammar. Ammar asked the enquirer: "Has such a problem taken shape actually?" The enquirer replied in the negative. Then Ammar said: "Whenever such a problem actually crops up we shall solve it".
Sense of responsibility in the matter of giving judgement is observed only by those who fall in the category of Siddiqin (very truthful), because such persons believe in accounting and weighing every word, and just as they avoid indecent deeds they also refrain from uttering improper words, and fear lest they should give an incorrect reply and thus commit a sin.
For this reason Ammar did not give a definite reply to any doubtful problem, except when necessary, so that he might remain immune from mistake.
One of the most evident and attractive manifestations of Ammar's piety was perhaps that instead of reciting the Friday sermon he used to recite Surah Yasin. 
In order to explain the matter it may be pointed out that as Arnmar was living at a time when he did not look at the government of the time favourably and had not expressed an agreeable opinion about it, and in the meantime wanted to say something which was not opposed to the establishment of security and contrary to the order of the day, he used to recite the Qur'an instead of delivering a sermon.
It is evident that in the circumstances the safest and most correct policy was to adopt this method, because Friday sermon had been invented with a view to strengthening and supporting the government of the day and the preacher was to pray for the safety of the ruler or the Caliph. However, Ammar had been appointed as governor at a time when in his heart of hearts he was not in favour of the government. In the circumstances if he had prayed for the government of the time it would have amounted to his playing with his faith and if he had said anything against the government he would have plunged himself into trouble. To get rid of this deadlock he had definitely no alternative except that instead of reciting the Friday sermon he should have recited one of the Surahs of the Holy Qur'an, especially Surah Yasin.
In any case it is not known whether this sermon had anything to do with the removal of Ammar from his office. However, what is undeniable and cannot be doubted is that the Caliph was not happy with Ammar's reciting this sermon. It was especially so, because he was suspicious about the members of the family of Bani Hashim and their wellwishers, and did not entrust them any duty lest the people should incline towards Bani Hashim and consequently troubles should arise.
We are not aware whether or not this Friday sermon affected the Ammar's position in the eyes of the Caliph and made him disappointed. However, Ibn Sa'd quotes another story which may not perhaps be irrelevant to quote here. He says: "One day Mutarraf arrived in Kufa and sat in the shop of a resident of that city. At that time another man was also sitting by the side of the shopkeeper, and a tailor, who was busy sewing a dress of sable fur or fox fur, said.
"……….I said: Have you not seen that Ali did this and that?" Mutarraf narrated:
"Then that very man said: "O libertine! Do you mean the Commander of the Faithful?"
"The shopkeeper said to me: "O pious man! Don't take offence at the words of this man who is my guest".
Mutarraf: "I understood at once that it was Ammar". As you can observe this story is brief and ambiguous.
However, the former attachments and inclinations of Ammar make the matter clear and remove ambiguity to some extent.
Suppose that the newcomer had mentioned the act attributed to Ali in good terms or suppose that he had explained it in bad terms. In either case his praise or vilification would have been equal in the face of the clear reply given by Ammar. And again if the newcomer had praised Ali and Ammar had felt from this praise that the speaker meant to attack the Caliph (and this meaning would have been more conformable to the circumstances of the time) and had criticized the speaker and shouted upon him this criticism and shouting would have meant mistrust on the part of Ammar, indicating that he considered the speaker to be a spy who wanted to find out his views after his appointment to the governorship of Kufa. And there is a great probability of the fact that the environments of Ammar necessitated that he should have been awake and careful on such occasions. And if we suppose that the new comer was speaking about Ali in a manner which was not liked by Ammar (and we do not imagine that the situation was such) Ammar's reply should be regarded to be a reflection of his inclination and heart felt attachment to Ali. In any case the meaning of this action for drawing the attention of the new-comer about the conduct of the Caliph is not free from criticism and he deserved to hear such a reply.
What is undeniable is that during the period of his appointment as Governor of Kufa Ammar as faced with a very difficult test and he carried out his duties with firm faith and special care. In any case it can be understood very well as to what the real talk had been.
Nevertheless it is possible that in this incident the object was to discover the views of Ammar about the Caliph after Umar. However the attachment of Ammar with Ali during the period of his governorship is considered to be one of the factors which led to the prevalence of Shi'aism in Kufa during the later period.
Ibn Sa'd in Tabaqat has narrated another story which is not less obscure and ambiguous than the foregoing one. He says: "Someone told tales about Ammar in the presence of Umar and this news also reached Ammar. On hearing this Ammar raised his hands and said: "O Lord! Give strength in the world to the person who has slandered me and leave him without punishment".
Self-purification and godliness are evident from these words. However, I have another point in view. The historians and traditionalists have not expressed a clear opinion about this incident and have not clarified the matter about which the tale-bearer spoke to the Caliph against Ammar.
Did Ammar take bribes? Did he commit adultery? Did he drink wine? Did he indulge in nepotism? Did he do anything unlawful? And did he contravene rules and regulations? Evidently Ammar was too prudent and careful to do such things and none of these acts could be ascribed to him.
In that event it may be asked: What art did the tale bearers and backbiters possibly employ? Of course, a definite reply to this question cannot be given, but it is not improbable that the talebearing about Ammar might be related to his petty activities because the Caliph was very much annoyed on account of those activities and could not see his agents and appointees in such a condition, And it is also not improbable that the curse, uttered by Ammar might have created further misgivings although the sense, denial, and refutation is perceptible and patent in Ammar's curse.
What actually happened is not known to us. However, the point which is proved and admitted is that after the occurrence of these ambiguous and brief events Umar removed Ammar from the governorship of Kufa. History does not specify the reason of Ammar's removal. However, at the same time, Umar retained Abdullah bin Mas'ud, who was Ammar's minister. The only excuse about Ammar's removal however, is stated to be that Ammar had been removing the appointees so soon. 
On having been relieved of the governorship of Kufa Ammar went to Madina. Umar asked him: "Did you feel unhappy on account of my having removed you from office?" Ammar replied explicitly and firmly: "The fact is that I didn't like my appointment nor my removal".
To sum up, it may be said that the relations between these two persons were such that in their heart of hearts they did not hold a favourable opinion about each other.
After having been relieved of his office Ammar dedicated himself to worship and good deeds and in the meantime succeeded along with his other brothers in acquiring knowledge and wisdom from Ali. He was busy guiding and educating the people on the basis of goodness and truth till the time when the disturbance during the last part of Uthman's rule took place.
 While confirming that the Second Caliph acted justly to the extent it was possible for him, Abdul Fatta Abdul Maqsud, a professor in Fawad University, Cairo says in his book entitled ‘Al Imam Ali bin Abi Talib’ "During the last years of Umar's rule the foundation of collective justice in Islam was shaken owing to his allowing larger shares from the public treasury to those who had embraced Islam earlier. During the last year of his rule he decided to retrace his steps, but death did not give him respite to do so". He also says: "The fire which he kindled in this regard did not provide heat to him but burnt Uthman ".
 It was on account of this policy that the contemporary scholar Prof. Abdullah Alaili says in his book entitled, Ashi'ah min Hayatil Husayn. "Till the end of Uthman's rule not even one Hashimite was appointed to a government post".
 Tabaqat Ibn Sa'd.
 It is evident that a resolute and nobleminded person like Ammar cannot give any opportunity to perverted appointees and removes them as soon as he finds them to be inefficient. And is there any virtue greater than this?