In his book "Lives of Successors of Muhammad", Washington Irving estimates the achievements of Umar in the following terms:
"The whole history of Umar shows him to have been a man of great powers of mind, inflexible integrity and rigid justice. He was more than any one else the founder of the Islamic empire; confirming and carrying out the inspirations of the Prophet; aiding Abu Bakr with his counsels during his brief Caliphate; and establishing wise regulations for the strict administration of the law throughout the rapidly-extending bounds of the Muslim conquests. The rigid hand which he kept upon his most popular generals in the midst of their armies, and in the most distant scenes of their triumphs, gives signal evidence of his extra-ordinary capacity to rule. In the simplicity of his habits, and his contempt for all pomp and luxury, he emulated the example of the Prophet and Abu Bakr. He endeavoured incessantly to impress the merit and policy of the same in his letters to his generals. 'Beware' he would say of Persian luxury both in food and raiment. Keep to the simple habits of your country, and Allah will continue you victorious; depart from them and He will reverse your fortunes'. It was his strong conviction of the truth of this policy which made him so severe in punishing all ostentatious style and luxurious indulgence in his officers. Some of his ordinances do credit to his heart as well as his head. He forbade that any female captive who had borne a child should be sold as a slave. In his weekly distributions of the surplus money of his treasury, he proportioned them to the wants, not the merits of the applicants. 'God' said he, 'has bestowed the good things of this world to relieve our necessities, not to reward our virtues: those will be rewarded in another world'."
In his book "The Caliphate, its Rise, Decline and Fall" Sir William Muir says as follows about Umar:
"Umar's life requires but few lines to sketch. Simplicity and duty were his guiding principles; impartiality and devotion the leading features of his administration. Responsibility so weighed upon him that he was heard to exclaim 'O that my mother had not borne me; would that I had been this stalk of grass instead!' In early life, of a fiery and impatient temper, he was known, even in the later days of the Prophet, as the stern advocate of vengeance. Ever ready to unsheathe the sword, it was he who at Badr advised that the prisoners should be put to death. But age, as well as office, had now mellowed this asperity. His sense of justice was strong. And except it be the treatment of Khalid, whom according to some accounts, he pursued with an ungenerous resentment, no act of tyranny or injustice is recorded against him; and even in this matter, his enmity took its rise in Khalid's unscrupulous treatment of fallen foe. The choice of his captains and governors was free from favouritism and (Al-Mughira and Ammar excepted) singularly fortunate. The various tribes and bodies in the empire, representing interests the most diverse, reposed in his integrity implicit confidence, and his strong arm maintained the discipline of law and empire. . . Whip in hand he would perambulate the streets and markets of Madina, ready to punish slenders on the spot; and so the proverb Umar's whip is more terribe than another's sword'. But with all this he was tender hearted, and numberless acts of kindness are recorded of him, such as relieving the wants of the widows and the fatherless."
In his classical work "The Decline and Fall of the Roman Empire, Gibbon refers to Umar in the following terms:
"Yet the abstinence and humility of Umar were not inferior to the virtues of Abu Bakr: his food consisted of barley bread or dates; his drink was water; he preached in a gown that was torn or tattered in twelve places; and a Persian satrap, who paid his homage as to the conqueror, found him asleep among the beggars on the steps of the mosque of Muslims. Economy is the source of liberality, and the increases of the revenue enabled Umar to establish a just and perpetual reward for the past and present services of the faithful. Careless of his own emolument, he assigned to Abbas, the uncle of the Prophet, the first and most ample allowance of twenty-five thousand dirhams of pieces of silver. Five thousand were allotted to each of the aged warriors? The relics of the field of Badr and the last and the meanest of the companions of Mohammad was distinguished by the annual reward of three thousand pieces. Under his reign and that of his predecessors, the conquerors of the East were the trusty servants of God and the people; the mass of public treasure was consecrated to the expenses of peace and war; a prudent mixture of justice and bounty maintained the discipline of the Saracens, and then united, by a rare felicity, the despatch and execution of despotism with the equal and frugal maxims of a republican government."
In his book "History of the Arabs" Professor Philip K. Hitti has assessed the achievements of Umar in the following terms:
"Simple and frugal in manner the energetic and talented Umar (634-644) who was of towering height, strong physique and bald headed, continued at least for some time after becoming the Caliph to support himself by trade and lived throughout his life in a style as unostentatious as that of a Bedouin Sheikh. In fact, Umar, whose name according to Muslim tradition is the greatest in early Islam after that of Mohammad, has been idolised by Muslim writers for his piety, justice and patriarchal simplicity and treated as the personification of all the virtues a Caliph ought to possess. His irreproachable character became an exemplar for all conscientious successors to follow. He owned, we are told, one shirt and one mantle only, both conspicuous for their patchwork, slept on a bed of palm leaves, and had no concern other than the maintenance of the purity of the faith, the upholding of justice and the ascendancy and security of Islam and the Arabians. Arabic literature is replete with anecdotes extolling Umar's stern character. He is said to have scourged his own son to death for drunkenness. Having in a fit of anger inflicted a number of stripes on a Bedouin who came seeking his succour against an oppressor, the Caliph soon repented and asked the Bedouin to inflict the same number on him. But the latter refused. So Umar retired to his home with the following soliloquy: 'O son of Al-Khattab humble thou wert and Allah has elevated thee, thou went astray, and Allah hath guided thee; thou were weak, and Allah hath strengthened thee. Then He caused thee to rule over the necks of thy people, and when one of them came seeking thy aid thou didst strike him! What wilt thou have to say to thy Lord when thou presentest thyself before Him'. The one who fixed the Hijrah as the commencement of the Muslim era, presided over the conquest of large portions of the then known world, instituted the state register and organised the government of the new empire, met a tragic and sudden death at the very zenith of his life when he was struck down by the poisoned dagger of a Christian Persian slave in the midst of his own congregation."
"The Encyclopaedia Britannica" remarks about Umar:
"To Umar's ten years' Caliphate belong, for the most part, the great conquests. He himself did not take the field, but remained in Madina; he never, however, suffered the reins to slip from his grasp, so powerful was the influence of his personality and the Muslim community of feeling. His political insight is shown by the fact that he endeavoured to limit the indefinite extension of Muslim conquest, to maintain and strengthen the national Arabian character of the commonwealth of Islam; also by making it his foremost task to promote law and order in its internal affairs. The saying with which he began his reign will never grow antiquated: 'By God, he that is weakest among you shall be in my eye the strongest, until I have vindicated for him his rights; he that is strongest I will treat as the weakest, until he complies with the law'. It would be impossible to give a better general definition of the function of the State."