Under Umar, the caliphate expanded at an unprecedented rate, ruling the Sasanian Empire and more than two-thirds of the Byzantine Empire. His attacks against the Sasanian Empire resulted in the conquest of Persia in fewer than two years
Umar was born in Mecca to the Banu Adi clan, which was responsible for arbitration among the tribes. He is said to have belonged to a middle-class family. In his youth he used to tend to his father's camels in the plains near Mecca. Despite literacy being uncommon in pre-Islamic Arabia, Umar learned to read and write in his youth. According to the tradition of Quraish, while still in his teenage years, Umar learned martial arts, horse riding and wrestling. He was tall, physically powerful and a renowned wrestler. He was also a gifted orator who succeeded his father as an arbitrator among the tribes.
In 610, Prophet Muhammad started preaching the message of Islam. Umar opposed Islam and he even threatened to kill Muhammad. He resolved to defend the traditional polytheistic religion of Arabia. He recommended Muhammad's death. He firmly believed in the unity of the Quraish and saw the new faith of Islam as a cause of division and discord.
The story of Umar's conversion was recounted in Ibn Ishaq's Sīrah. On his way to murder Muhammad, Umar met his best friend Na'eem bin Abdullah who had secretly converted to Islam but had not told Umar.
According to Muhammad al-Bukhari, "While 'Umar was at home in a state of fear, there came Al-'As bin Wail As-Sahmi Abu 'Amr who was an open enemy to Muhammad, wearing an embroidered cloak and a shirt having silk hems. He was from the tribe of Bani Sahm who were our allies during the pre-Islamic period of ignorance. Al-'As said to 'Umar 'What is wrong with you?' He said, 'Your people claim that they would kill me if I become a Muslim.' Al-'As said, 'Nobody will harm you after I have given protection to you.'
Umar was a man of his word and attempted to do what he said. So in order to divert his attention, Nuaim told him to inquire about his own house where his sister and her husband had converted to Islam.
He washed his body upon his sister's request and then began to read the verses that were: Verily, I am Allah: there is no God but Me; so serve Me (only), and establish regular prayer for My remembrance (Quran 20:14).
Umar then went to Muhammad with the same sword he intended to kill him with and accepted Islam in front of him and his companions. Umar was 27 years old when he accepted Islam.
Most Muslims migrated at night fearing resistance from Quraish at their migration, but Umar is reported to have left openly during the day saying: "Any one who wants to make his wife a widow and his children orphans should come and meet me there behind that cliff."
When Muhammad died on 8 June 632 Umar initially disbelieved that he was dead It is said that Umar promised to strike the head of any man who would say that Muhammad died.
Umar was not a very popular figure among the notables of Medina, accordingly succession of Umar was initially discouraged by companions of Abu Bakr. Nevertheless, Abu Bakr decided to make Umar his successor.
Even though almost all of the Muslims had given their pledge of loyalty to Umar, he was rather more feared than loved. According to Muhammad Husayn Haykal, the first challenge for Umar was to win over his subjects and members of Majlis al Shura. Umar was a gifted orator, and he would use his ability to get a soft corner in the hearts of people. Muhammad Husayn Haykal wrote that Umar's stress was on the well-being of poor and underprivileged people.
Umar was a pioneer in some affairs: 1. Umar was the first to introduce the public ministry system, where the records of officials and soldiers were kept. He also kept a record system that had the messages he sent to Governors and heads of states. 2. He was the first to appoint police forces to keep civil order. 3. He was the first to discipline the people when they became disordered.