Umar was the second Rashidun Caliph and reigned during 634–644. Umar ibn Al-Khattāb caliphate is notable for its vast conquests. Aided by brilliant field commanders, he was able to incorporate present-day Iraq, Iran, Azerbaijan, Armenia, Georgia, Syria, Jordan, Palestine, Lebanon, Egypt, and parts of Afghanistan, Turkmenistan and south western Pakistan into the Caliphate. During his reign, the Byzantines lost more than three fourths of their territory and in Persia, Umar ibn Al-Khattāb became the king (ruler) of Iran after the fall of the Sassanid Empire.A Restatement of the History of Islam and Muslims on Al-Islam.org
Historians estimate more than 4,050 cities were conquered during the reign of Umar ibn Al-Khattāb.
Military conquests of Umar ibn Al-Khattāb era are:
Conquest of the Levant and Upper Iraq (634–638)
Muslim forces invaded the neighboring Eastern Roman Empire in 634 soon after the Conquest of Iraq in 633 during the reign of Caliph Abu Bakr. Damascus fell in September 634 and Emesa in March 635. In the year 635, Emperor Heraclius allied with Sassanid Persian Emperor Yazdegerd III on the latter's request to crush the Muslim power. A plan was agreed to launch a massive counterattack against Muslims in Iraq and Syria simultaneously to force them to retreat to Arabia where they could be dealt with later, either through invasion or economic blockade.
The Rashidun caliphate dealt the Byzantines crushing defeats at the battles of Ajnadayn and Fahl. These significantly reduced the capacity of Byzantine army to operate in southern Syria and, according to historian Ross Burns, the massive losses from these battles practically wiped out the "southern Damascus shield", the Imperial forces which protected southern Syria.
Caliph Umar ibn Al-Khattāb successfully confronted the alliance by putting pressure on the Byzantines, while engaging Yazdegerd III in negotiations. This rendered the alliance weak and a would-be decisive plan was aborted. The Byzantine forces were decisively defeated in Battle of Yarmouk in August 636, while in the Iraqi theater the Persian army was defeated in the Battle of Qadisiyyah three months later in November 636.
Defense of Emesa and Conquest of Upper Mesopotamia
After the defeat in the Battle of Yarmouk, Heraclius mounted a counterattack operation in Syria. Heraclius sought help from the Christian Arab tribes of al-Jazira, which came mostly from two cities along the Euphrates river, Circesium and Hīt. The tribes mustered a large army and marched against Emesa where Abu Ubaydah had set up his military headquarters. As the Christian Arabs contingents besieged Emesa, Khalid appealed to Abu Ubaydah to be allowed to lead a sally outside the wall. However, Abu Ubaidah decided it would be better should to wait for reinforcements The sieges of Circessium and Hit by troops under Iyadh occurred simultaneously as the siege of Emesa. At first the Muslims faced difficulty at Hit as the defenders dug a moat around the city, but eventually the Muslim army was able to penetrate it. Meanwhile, Circesium was captured from the Byzantines without resistance by a Muslim army commanded by Habib. Though many Muslim sources state this occurred in 637, Maximillan Streck stated it is more likely to have happened in 640. As response to the siege of Emesa, Iyad was tasked by caliph Umar through his superior, Abu Ubaydah, to invade Al-Jazira.
When Abu Ubayda died in 639, Caliph Umar appointed Iyad in his place as the ʿamal (governor) of Hims, Qinnasrin (Chalcis) and al-Jazira with instructions to conquer the latter territory from its Byzantine commanders because they had refused to pay the tributes promised to the Muslims in 638. By the time Iyad was given his assignment, all of Syria had been conquered by the Muslims, leaving the Byzantine garrisons in al-Jazira isolated from the empire. In August 639, Iyad led a 5,000-strong army toward Raqqa (Kallinikos) in al-Jazira and raided the city's environs. He encountered resistance from its defenders, prompting him to withdraw and send smaller units to make raids around Raqqa, seizing captives and harvests. After five or six days of these raids, Raqqa's patrician negotiated the surrender of the city to Iyad. According to historian Michael Meinecke, Iyad captured the city in 639 or 640. After Raqqa, Iyad proceeded toward Harran, where his progress was stalled. He diverted part of his army to Edessa, which ultimately capitulated after negotiations. Iyad then received Harran's surrender and dispatched Safwan ibn Mu'attal al-Sulami and his own kinsman Habib ibn Maslama al-Fihri to seize Samosata, which also ended in a negotiated surrender after Muslim raiding of its countryside. By 640, Iyad had successively conquered Saruj, Jisr Manbij and Tell Mawzin. Before the capture of Tell Mawzin, Iyad attempted to take Ras al-Ayn, but retreated after stiff resistance. Later, he dispatched Umayr ibn Sa'd al-Ansari to take the city. Umayr first assaulted the rural peasantry and seized cattle in the town's vicinity. The inhabitants barricaded inside the walled city and inflicted heavy losses on the Muslim forces, before ultimately capitulating. About the same time, Iyad besieged Samosata in response to a rebellion, the nature of which is not specified by al-Baladhuri, and stationed a small garrison in Edessa after the city's inhabitants violated their terms of surrender.
The counter sieges carried out by Iyad did not stop with Circesium and Hit, as Iyadh further sent Walid ibn Uqba to subdue the fortresses of the tribe of Rabi'a and Tanukhid. After Samosata, al-Baladhuri, states that Iyad subdued a string of villages "on the same terms" as Edessa's surrender. Between the end of 639 and December 640, Iyad and his lieutenants subdued, in succession, Circesium (al-Qarqisiya), Amid, Mayyafariqin, Nisibin, Tur Abdin, Mardin, Dara, Qarda and Bazabda. In the case of Raqqa (Kallinikos to the Byzantines), the peasants outside the city walls were defended by the Arab Christian nomads. There, the Muslim forces compelled the city's leaders, facing the prospect of starvation, to surrender within five or six days. Since its capture by Muslims it has figured in Arabic sources as al-Raqqah. Meanwhile, caliph Umar personally led reinforcements from Medina, which joined with reinforcements from Iraq led by al-Qa'qa. Realizing the threat from the combined forced, along with Iyad's invasions of their homeland in Jazira, the Christian Arabs immediately abandoned the siege and hastily went to defend their homeland. By the time the Christian Arab left, Khalid and his mobile guard had been reinforced by 4000 soldiers under Qa'qa from Iraq, and were now given permission by Abu Ubaydah to came out of the fort and pursue the enemy.
After the successful defense of Emesa, which done simultaneously with the conquest of upper Mesopotamia, the Muslim armies split up. Shurhabil and Amr's corps moved south to capture Palestine, while Abu Ubaidah and Khalid, with a relatively larger corps, moved north to conquer Northern Syria. While the Muslims were occupied at Fahl, Heraclius, sensing an opportunity, quickly sent an army under General Theodras to recapture Damascus, where a small Muslim garrison was left. Shortly thereafter, the Muslims, having just won the Battle of Fahl, moved to Emesa. In the meantime, the Byzantine army split in two, one deployed at Maraj al Rome (Beqaa Valley) led by Schinos; the other, commanded by Theodras, stationed to the west of Damascus (Al-Sabboura region). During the night, Theodras advanced to Damascus to launch a surprise attack. Khalid's spy informed him about the move and Khalid moved quickly towards Damascus with his mobile guard. While Abu Ubaidah fought and defeated the Roman army under Schinos, Khalid attacked and defeated Theodras. A week later, Abu Ubaida himself moved towards Baalbek, where the great Temple of Jupiter stood. In May 636, Heliopolis surrendered to the Muslims after little resistance and agreed to pay tribute. Abu Ubaidah sent Khalid straight towards Emesa.
Siege of Jerusalem and Asia Minor conquest
Muslim victories pertinently ended Byzantine rule south of Anatolia, and Jerusalem fell in April 637 after a prolonged siege, Umar personally came to receive the key to the city by the Greek Orthodox patriarch, Sophronius, and was invited to offer prayers at the Church of the Holy Sepulchre. Umar chose to pray some distance from the Church, so as not to endanger its status as a Christian temple. Fifty-five years later, the Mosque of Omar was constructed on the site where he prayed.For one version of 'Umar's speech to the people after the surrender of Jerusalem, see . After the fall of Jerusalem, Umar permitted Jews to practice their religion freely and live in Jerusalem.
The conquest of Levant was completed in 637 after the last relentless resistance by Byzantines at Battle of Iron bridge, which resulted in Muslim occupation of Antioch, capital of eastern zone of Byzantine Empire in October 637. Emperor Heraclius attempt to capture northern Syria in 638, with the aid of Christian Arabs of Jazira, prompted Muslims to invade Jazira (Mesopotamia) in 638 and captured Marash in the Siege of Germanicia, securing the eastern flank of Syria from Byzantine attacks in future, soon after the occupation of Jazira, Muslim columns marched north in Anatolia, invaded and plundered Byzantine provinces of Armenia, these were however only preemptive attacks on Armenia to eliminate all Byzantine presence north of Syria, Armenia was annexed in 643 during the Conquest of Persian Empire. These preemptive attack resulted in the creation of a buffer zone or no man's land in south-eastern Anatolia and Armenia, which would eventually evolve into the al-'Awasim. It was exactly what Umar wanted,kegri as he is quoted saying
The Byzantine empire already exhausted after major defeats in Yarmouk and Northern Syria was left vulnerable to Muslims' attacks and its very existence in Anatolia was threatened. Umar apparently was not interested in occupation of Anatolia, it had a cold and mountainous terrain with no economic incentives and his main purpose of these conquests was capturing Jerusalem, soon after the occupation of Byzantine Armenia, the time when chaos was at its peak in Byzantium, Umar had already rejected Khalid and Abu Ubaidah's proposal of invading Anatolia. Moreover, Umar, due to his strong desire to consolidate his rule in the conquered land and owing to his non-offensive policy left the remaining Byzantine empire on its own. The situation was a stalemate, Umar had power but not desire to cross into Byzantine Empire, and Emperor Heraclius had desire but not left powerful enough to roll back his former rich provinces. For the security of northern Syria, Umar issued orders for annual raids into Byzantine territories in Anatolia and Muslims raided as far as Phrygia.
Conquest of Africa (640–643)
After losing the Levant, the economic lifeline and main source of manpower of Byzantines and Armenia, Emperor Heraclius was left incapable of any military come back. As a result, he focused on consolidating his power in Egypt. During his visit to Syria in 637 to receive the surrender of Jerusalem, Amr ibn al-Aas tried to convince Umar to invade Egypt, but Umar rejected on the grounds that Muslim rule in Syria was still not firm. After the great plague in 639, Umar paid another visit to Syria and was again persuaded by Amr to invade Egypt. Amr convinced Umar that Byzantine influence in Egypt was a continuous threat to Muslim rule in Palestine and that Egypt was a rich land that could provide Muslims with immense wealth, economical stability as well as a strategic location for trade with North Africa and Mediterranean. Initially hesitant, Umar rejected the proposal and is reported to have said "Life of my one soldier is dearer to me than a million Dirham.”