The Rashidun Caliphate was the first of the four major caliphates established after the death of the Islamic prophet Muhammad. It was ruled by the first four successive caliphs (successors) of Muhammad after his death in 632 AD (AH 11). These caliphs are collectively known in Sunni Islam as the Rashidun, or “Rightly Guided” caliphs. This term is not widely used in Shia Islam as Shia Muslims do not consider the rule of the first three caliphs as legitimate.
The Rashidun Caliphate is characterized by a twenty-five year period of rapid military expansion, followed by a five-year period of internal strife. The Rashidun Army at its peak numbered more than 100,000 men. By the 650s, the caliphate in addition to the Arabian Peninsula had subjugated the Levant, to the Transcaucasus in the north; North Africa from Egypt to present-day Tunisia in the west; and the Iranian plateau to parts of Central Asia and South Asia in the east.
The caliphate arose out of the death of Prophet Muhammad in 632 CE and the subsequent debate over the succession to his leadership. Abu Bakr, a close companion of Muhammad from the Banu Taym clan, was elected the first Rashidun leader and began the conquest of the Arabian Peninsula. He ruled from 632 to his death in 634. Abu Bakr was succeeded by Umar, his appointed successor from the Banu Adi clan, who continued the conquest of Persia, eventually leading to the fall of the Sassanid Empire in 651. Umar was assassinated in 644 and was succeeded by Uthman, who was elected by a six-person committee arranged by Umar. Under Uthman began the conquest of Armenia, Fars and Khorasan. Uthman was assassinated in 656 and succeeded by Ali, who presided over the civil war known as the First Fitna (656–661). The war was primarily between those who supported Uthman’s cousin and governor of the Levant, Muawiyah, and those who supported the caliph Ali. The civil war permanently consolidated the divide between Sunni and Shia Muslims, with Shia Muslims believing Ali to be the first rightful caliph and Imam after Muhammad. A third faction in the war supported the governor of Egypt, Amr ibn al-As. The war was decided in favour of the faction of Muawiyah, who established the Umayyad Caliphate in 661.
Conquest of the Sasanian Empire
The first Islamic invasion of the Sasanian Empire, launched by Caliph Abu Bakr in 633, was a swift conquest, taking only four months. Abu Bakr sent his general, Khalid ibn Walid, to conquer Mesopotamia after the Ridda wars. After entering Iraq with his army of 18,000, Khalid won decisive victories in four consecutive battles: the Battle of Chains, fought in April 633; the Battle of River, fought in the third week of April 633; the Battle of Walaja, fought in May 633 (where he successfully used a pincer movement), and the Battle of Ullais, fought in mid-May of 633. In the last week of May 633, the capital city of Iraq fell to the Muslims after initial resistance in the Battle of Hira.
After resting his armies, Khalid moved in June 633 towards Al Anbar, which resisted and was defeated, and eventually surrendered after a siege of a few weeks in July 633. Khalid then moved towards the south and conquered the city of Ein ul Tamr in the last week of July 633. By now, almost the whole of Iraq was under Islamic control. Khalid received a call for help from Daumat-ul-jandal in Northern Arabia, where another Muslim general, Iyad ibn Ghanm, was trapped among the rebel tribes. Khalid diverted there and defeated the rebels in the Battle of Daumat-ul-jandal in the last week of August 633. Returning from Arabia, he received news that a large Persian army was assembling. Within a few weeks, he decided to defeat them piecemeal in order to avoid the risk of defeat by a large unified Persian army. Four divisions of Persian and Christian Arab auxiliaries were present at Hanafiz, Zumiel, Sanni and Muzieh.
In November 633, Khalid divided his army into three units, and attacked these auxiliaries one by one from three different sides at night, starting with the Battle of Muzieh, then the Battle of Sanni, and finally the Battle of Zumail. These devastating defeats ended Persian control over Iraq. In December 633, Khalid reached the border city of Firaz, where he defeated the combined forces of the Sasanian Persians, Byzantines and Christian Arabs in the Battle of Firaz. This was the last battle in his conquest of Iraq.
Khalid then left Mesopotamia to lead another campaign in Syria against the Byzantine Empire, after which Mithna ibn Haris took command in Mesopotamia. The Persians once again concentrated armies to regain Mesopotamia, while Mithna ibn Haris withdrew from central Iraq to the region near the Arabian desert to delay war until reinforcement came from Medina. Umar sent reinforcements under the command of Abu Ubaidah Saqfi. With some initial success, this army was finally defeated by the Sasanian army at the Battle of the Bridge in which Abu Ubaid was killed. The response was delayed until after a decisive Muslim victory against the Romans in the Levant at the Battle of Yarmouk in 636. Umar was then able to transfer forces to the east and resume the offensive against the Sasanians. Umar dispatched 36,000 men along with 7500 troops from the Syrian front, under the command of Sa`d ibn Abī Waqqās against the Persian army. The Battle of al-Qādisiyyah followed, with the Persians prevailing at first, but, on the third day of fighting, the Muslims gained the upper hand. The legendary Persian general Rostam Farrokhzād was killed during the battle. According to some sources, the Persian losses were 20,000, and the Arabs lost 10,500 men.
Following this Battle, the Arab Muslim armies pushed forward toward the Persian capital of Ctesiphon (also called Madā’in in Arabic), which was quickly evacuated by Yazdgird after a brief siege. After seizing the city, they continued their drive eastwards, following Yazdgird and his remaining troops. Within a short span of time, the Arab armies defeated a major Sasanian counter-attack in the Battle of Jalūlā’, as well as other engagements at Qasr-e Shirin, and Masabadhan. By the mid-7th century, the Arabs controlled all of Mesopotamia, including the area that is now the Iranian province of Khuzestan. It is said that Caliph Umar did not wish to send his troops through the Zagros mountains and onto the Iranian plateau. One tradition has it that he wished for a “wall of fire” to keep the Arabs and Persians apart. Later commentators explain this as a common-sense precaution against over-extension of his forces. The Arabs had only recently conquered large territories that still had to be garrisoned and administered. The continued existence of the Persian government was, however, incitement to revolt in the conquered territories and unlike the Byzantine army, the Sasanian army was continuously striving to regain their lost territories. Finally, Umar pressed forward, which eventually resulted in the wholesale conquest of the Sasanian Empire. Yazdegerd, the Sasanian king, made yet another effort to regroup and defeat the invaders. By 641 he had raised a new force, which made a stand at the Battle of Nihawānd, some forty miles south of Hamadan in modern Iran. The Rashidun army, under the command of Umar’s appointed general Nu’man ibn Muqarrin al-Muzani, attacked and again defeated the Persian forces. The Muslims proclaimed it the Victory of Victories (Fath alfotuh), as it marked the End of the Sasanians, shattering the last strongest Sasanian army.
Yazdegerd was unable to raise another army and became a hunted fugitive. In 642 Umar sent the army to conquer the remainder of the Persian Empire. The entirety of present-day Iran was conquered, followed by Greater Khorasan (which included the modern Iranian Khorasan province and modern Afghanistan), Transoxania, and Balochistan, Makran, Azerbaijan, Dagestan (Russia), Armenia and Georgia; these regions were later re-conquered during Uthman’s reign with further expansion into the regions which were not conquered during Umar’s reign; hence, the Rashidun Caliphate’s frontiers in the east extended to the lower river Indus and north to the Oxus River.