After his conversion to Islam, he turned his sword on Muhammad's enemies. When prisoners were captured in war, Abu Bakr advised the prophet to have mercy while Omar advised him to slaughter them. Often, Omar was frustrated by Muhammad's leniency towards enemies. He disagreed about what he saw as concessions made toward foes. However, under Muhammad's guidance, Omar replaced his vengeful nature with devotion to God. On many occasions when people used disrespectful language toward the prophet, Omar hastened to draw his sword but was stopped by Muhammad, and when Omar first learned of Muhammad's death, he thought it was a malicious lie. Many were fearful that when Abu Bakr appointed Omar as the next caliph, he would be hard on his enemies. However, Abu Bakr convinced them that Omar's becoming caliph would bring out his mercy. Abu Bakr was right.
By the beginning of Omar's rule, Islam was already at war with the Byzantine and Persian empires. By the end of his rule, Islam's dominion stretched across the Middle East and included areas populated by a variety of races, languages, and faiths. However, throughout this period of geographic expansion, Omar never sought riches, nor did he become a fanatic bent on Islamic world domination. For example, after the conquest of Iraq, one of Omar's generals asked permission to pursue the Persians into their homeland, but Omar replied, "I desire that between Mesopotamia and the countries beyond, the hills shall be a barrier so that the Persians shall not be able to get at us, nor we at them. The plain of al-Iraq suffices for our wants. I would prefer the safety of my people to thousands of spoils and further conquest."
When further conflict with the Persians ensued and some Muslims requested that Omar reverse his policy toward expansion into Persia, he replied, "What is the cause that these Persians persistently break faith and rebel against us? Maybe you treat them harshly." Eventually Omar conceded his position, and the entire Persian Empire fell before the Muslims. When the vast treasures of the conquests were brought before him, he wept, because he saw in these treasures seeds of destruction in the Islamic community.
As ruler of a mighty nation, Omar lived in hermitlike austerity, taking nothing for himself. He often acted as a commoner, working with his own hands to help the poor and hungry, and once, while traveling from Jerusalem to Arabia, he took turns riding his camel with the single servant who had accompanied him. Furthermore and despite his reputation for harshness, Omar maintained a policy of tolerance toward other faiths. In Jerusalem, for example, he established a treaty guaranteeing the life, property, and complete freedom of religion to the people of Jerusalem. And when invited to pray in Jerusalem's Church of the Resurrection, he politely declined, saying, "Should we say our prayers here, Muslims might someday claim the right to erect a mosque in this place."
Even at his death, Omar remained a spiritual exemplar. Omar was stabbed mortally during prayer, but he remained calm and peaceful. When told that the attacker was a Christian, he thanked God that his murderer was not a Muslim, for that would have shattered the unity of the community. He died, then, totally committed to Islam and to the establishment of an Islamic community.