Battle of Yarmuk
Background: In 610, during the Byzantine–Sasanian War of 602–628, Heraclius became the emperor of the Byzantine Empire, after overthrowing Phocas. Meanwhile, the Sasanian Empire conquered Mesopotamia and in 611 they overran Syria and entered Anatolia, occupying Caesarea Mazaca (now Kayseri, Turkey). In 612, Heraclius managed to expel the Persians from Anatolia but was decisively defeated in 613 when he launched a major offensive in Syria against the Persians. Over the following decade, the Persians were able to conquer Palestine and Egypt. Meanwhile, Heraclius prepared for a counterattack and rebuilt his army.
In 622, Heraclius finally launched his offensive. After his overwhelming victories over the Persians and their allies in the Caucasus and Armenia, Heraclius launched a winter offensive against the Persians in Mesopotamia in 627, winning a decisive victory at the Battle of Nineveh, thus threatening the Persian capital city of Ctesiphon. Discredited by the series of disasters, Khosrow II was overthrown and killed in a coup led by his son Kavad II, who immediately sued for peace and agreed to withdraw from all occupied territories of the Byzantine Empire. Heraclius restored the True Cross to Jerusalem with a majestic ceremony in 629.
Meanwhile, there had been rapid political development in the Arabian Peninsula, where Muhammad had been preaching Islam and, by 630, had successfully annexed most of Arabia under a single political authority. When Muhammad died in June 632, Abu Bakr was chosen as caliph and his political successor. Troubles emerged soon after Abu Bakr's succession, and several Arab tribes openly revolted against him. He declared war against the rebels. In what became known as the Ridda wars of 632–633, Abu Bakr managed to defeat his opponents and unite Arabia under the central authority of the caliph at Medina.

Once the rebels had been subdued, Abu Bakr began a war of conquest, beginning with Iraq. His most brilliant general, Khalid ibn al-Walid, conquered Iraq in a series of successful campaigns against the Sassanid Persians. Abu Bakr's confidence grew, and once Khalid had established his stronghold in Iraq, Abu Bakr issued a call to arms for the invasion of Syria in February 634. The Muslim invasion of Syria was a series of carefully planned and well-co-ordinated military operations, which employed strategy, instead of pure strength, to deal with the Byzantine defensive measures.
The Muslim armies, however, soon proved to be too small to handle the Byzantine response, and their commanders called for reinforcements. Khalid was sent by Abu Bakr from Iraq to Syria with reinforcements and to lead the invasion. In July, the Byzantines were decisively defeated at Ajnadayn. Damascus fell in September, followed by the Battle of Fahl, in which the last significant garrison of Palestine was routed.
After Abu Bakr died in 634, his successor, Umar, was determined to continue the Caliphate's expansion deeper into Syria. Though previous campaigns led by Khalid had been successful, he was replaced by Abu Ubaidah. Having secured southern Palestine, Muslim forces now advanced up the trade route, and Tiberias and Baalbek fell without much struggle and conquered Emesa early in 636. The Muslims then continued their conquest across the Levant.

Byzantine counterattack: Having seized Emesa, the Muslims were just a march away from Aleppo, a Byzantine stronghold, and Antioch, where Heraclius resided. Seriously alarmed by the series of setbacks, Heraclius prepared for a counterattack to reacquire the lost regions. In 635 Yazdegerd III, the Emperor of Persia, sought an alliance with the Byzantine Emperor. Heraclius married off his daughter (according to traditions, his granddaughter) Manyanh to Yazdegerd III, to cement the alliance. While Heraclius prepared for a major offensive in the Levant, Yazdegerd was to mount a simultaneous counterattack in Iraq, in what was meant to be a well-coordinated effort. When Heraclius launched his offensive in May 636, Yazdegerd could not co-ordinate with the maneuver, probably owing to the exhausted condition of his government, and what would have been a decisive plan missed the mark.

Byzantine preparations began in late 635 and by May 636 Heraclius had a large force concentrated at Antioch in Northern Syria. The assembled Byzantine army contingents consisted of Slavs, Franks, Georgians, Armenians, Christian Arabs, Lombards, Avars, Khazars,  Balkans and Göktürks. The force was organized into five armies, the joint leader of which was Theodore Trithyrius. Vahan, an Armenian and the former garrison commander of Emesa, was made the overall field commander, and had under his command a purely Armenian army. Buccinator (Qanatir), a Slavic prince, commanded the Slavs and Jabalah ibn al-Aiham, king of the Ghassanid Arabs, commanded an exclusively Christian Arab force. The remaining contingents, all European, were placed under Gregory and Dairjan. Heraclius himself supervised the operation from Antioch. Byzantine sources mention Niketas, son of the Persian general Shahrbaraz, among the commanders, but it is not certain which army he commanded.
The Rashidun army was then split into four groups: one under Amr in Palestine, one under Shurahbil in Jordan, one under Yazid in the Damascus-Caesarea region and the last one under Abu Ubaidah along with Khalid at Emesa.
As the Muslim forces were geographically divided, Heraclius sought to exploit that situation and planned to attack. He did not wish to engage in a single pitched battle but rather to employ central position and fight the enemy in detail by concentrating large forces against each of the Muslim corps before they could consolidate their troops. By forcing the Muslims to retreat, or by destroying Muslim forces separately, he would fulfil his strategy of recapturing lost territory. Reinforcements were sent to Caesarea under Heraclius' son Constantine III, probably to tie down Yazid's forces, which were besieging the town. The Byzantine imperial army moved out from Antioch and Northern Syria in the middle of June 636.
The Byzantine imperial army was to operate under the following plan:

Jabalah's lightly armed Christian Arabs would march to Emesa from Aleppo via Hama and hold the main Muslim army at Emesa.
Dairjan would make a flanking movement between the coast and Aleppo's road and approach Emesa from the west, striking at the Muslims' left flank while they were being held frontally by Jabalah.
Gregory would strike the Muslims' right flank, approaching Emesa from the northeast via Mesopotamia.
Qanatir would march along the coastal route and occupy Beirut, from where he was to attack the weakly defended Damascus from the west to cut off the main Muslim army at Emesa.
Vahan's corps would act as a reserve and would approach Emesa via Hama.

Rashidun strategy: The Muslims discovered Heraclius' preparations at Shaizar from Byzantine prisoners. Alert to the possibility of being caught with separated forces that could be destroyed, Khalid called a council of war and advised Abu Ubaidah to pull the troops back from Palestine and Northern and Central Syria and concentrate the entire Rashidun army in one place. Abu Ubaidah ordered the concentration of troops in the vast plain near Jabiyah, as control of the area made cavalry charges possible and facilitated the arrival of reinforcements from Umar, so that a strong, united force could be fielded against the Byzantine armies. The position also benefited from close proximity to the Rashidun stronghold of Najd, if retreat became necessary. Instructions were also issued to return jizya (tribute) to people who had paid it.
However, once concentrated at Jabiyah, the Muslims were subject to raids from pro-Byzantine Ghassanid forces. Encamping in the region was also precarious as a strong Byzantine force was garrisoned in Caeseara and could attack the Muslim rear while they were held in front by the Byzantine army. On Khalid's advice the Muslim forces retreated to Dara'ah (or Dara) and Dayr Ayyub, covering the gap between the Yarmuk Gorges and the Harra lava plains, and established a line of camps in the eastern part of the plain of Yarmuk. This was a strong defensive position, and the maneuverers brought the Muslims and Byzantines into a decisive battle, which the latter had tried to avoid. During the maneuvers, there were no engagements except for a minor skirmish between Khalid's elite light cavalry and the Byzantine advance guard.

Battlefield: The battlefield lies in the plain of Jordanian Hauran, just southeast of the Golan Heights, an upland region currently on the frontier between Jordan and Syria, east of the Sea of Galilee. The battle was fought on the plain east of Raqqat stream ravine. That ravine joins the Yarmuk River, a tributary of the Jordan River, on its south.