Battle 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. The stream had very steep banks, ranging from 30 m (98 ft)–200 m (660 ft) in height. On the north is the Jabiyah road and to the east are the Azra hills although the hills were outside the actual field of battle. Strategically, there was only one prominence in the battlefield: a 100 m (330 ft) elevation known as Tel al Jumm'a, and for the Muslim troops concentrated there, the hill gave a good view of the plain of Yarmuk. The ravine on the west of the battlefield was accessible at a few places in 636 AD, and had one main crossing: a Roman bridge (Jisr-ur-Ruqqad) near Ain Dhakar Logistically, the Yarmuk plain had enough water supplies and pastures to sustain both armies. The plain was excellent for cavalry maneuvers.

Troop deployment: Most early accounts place the size of the Muslim forces between 36,000 and 40,000 and the number of Byzantine forces between 60,000 and 70,000 (This number has been estimated by taking into account the logistical situation of the Empire and with the view that they could never have mustered such troops when the Empire was at its apex but especially not with the especially weak and exhausted realm of 628 onwards).  Modern estimates for the sizes of the respective armies vary: some estimates for the Byzantine army are between 80,000 and 150,000, while other estimates are 15,000 to 20,000. Estimates for the Rashidun army are between 15,000 and 40,000. Original accounts are mostly from Arab sources, generally agreeing that the Byzantine army and their allies outnumbered the Muslim Arabs by a 2 to 1. The only early Byzantine source is Theophanes, who wrote a century later. Accounts of the battle vary, some stating it lasted a day, others six days.

Rashidun army: During a council of war, the command of the Muslim army was transferred to Khalid by Abu Ubaidah, Commander in Chief of the Muslim army.
After taking command, Khalid reorganized the army into 36 infantry regiments and four cavalry regiments, with his cavalry elite, the mobile guard, held in reserve. The army was organised in the Tabi'a formation, a tight, defensive infantry formation.
The army was lined up on a front of 12 kilometres (7.5 mi), facing west, with its left flank lying south on the Yarmuk River a mile before the ravines of Wadi al Allan began. The army's right flank was on the Jabiyah road in the north across the heights of Tel al Jumm'a, with substantial gaps between the divisions so that their frontage would match that of the Byzantine battle line at 13 kilometres (8.1 mi). The centre of the army was under the command of Abu Ubaidah ibn al-Jarrah (left centre) and Shurahbil bin Hasana (right centre). The left wing was under the command of Yazid and the right wing was under Amr ibn al-A's.
The centre, left and right wings were given cavalry regiments, to be used as a reserve for a counterattack if they were pushed back by the Byzantines. Behind the centre stood the mobile guard under the personal command of Khalid. If Khalid was too occupied in leading the general army, Dharar ibn al-Azwar would command the mobile guard. Over the course of the battle, Khalid would repeatedly make critical and decisive use of that mounted reserve.
Khalid sent out several scouts to keep the Byzantines under observation. In late July, Vahan sent Jabalah with his lightly armoured Christian-Arab forces to reconnoitre-in-force, but they were repulsed by the mobile guard. After the skirmish, no engagement occurred for a month.

Weaponry: Helmets used included gilded helmets similar to the silver helmets of the Sassanid empire. Mail was commonly used to protect the face, neck, and cheeks as an aventail from the helmet or as a mail coif. Heavy leather sandals, as well as Roman-type sandal boots, were also typical of the early Muslim soldiers. Armour included hardened leather scale or lamellar armour and mail armour. Infantry soldiers were more heavily armoured than horsemen. Large wooden or wickerwork shields were used. Long-shafted spears were used, with infantry spears being 2.5 m (8.2 ft) long and cavalry spears being up to 5.5 m (18 ft) long. Short infantry swords like the Roman gladius and Sassanid long swords were used; long swords were usually carried by horsemen. Swords were hung in baldrics. Bows were about 2 metres (6.6 ft) long when unbraced, similar in size to the famous English longbow. The maximum useful range of the traditional Arabian bow was about 150 m (490 ft). Early Muslim archers, while being infantry archers without the mobility of horseback archer regiments, proved to be very effective in defending against light and unarmoured cavalry attacks.

Byzantine army: A few days after the Muslims encamped at the Yarmuk plain, the Byzantine army, preceded by the lightly armed Ghassanids of Jabalah, moved forward and established strongly fortified camps just north of the Wadi-ur-Ruqqad.
The right flank of the Byzantine army was at the south end of the plains, near the Yarmuk River and about a mile before the ravines of Wadi al Allan began. The left flank of the Byzantines was at the north, a short distance before the Hills of Jabiyah began, and was relatively exposed. Vahan deployed the Imperial Army facing east, with a front about 13 kilometres (8.1 mi) long, as he was trying to cover the whole area between the Yarmuk gorge in the south and the Roman road to Egypt in the north, and substantial gaps had been left between the Byzantine divisions. The right wing was commanded by Gregory and the left by Qanatir. The centre was formed by the army of Dairjan and the Armenian army of Vahan, both under the overall command of Dairjan. The Byzantine regular heavy cavalry, the cataphract, was distributed equally among the four armies, each army deploying its infantry at the forefront and its cavalry as a reserve in the rear. Vahan deployed Jabalah's Christian Arabs, mounted on horses and camels, as a skirmishing force, screening the main army until its arrival.
Early Muslim sources mention that the army of Gregory had used chains to link together its foot soldiers, who had all taken an oath of death. The chains were in 10-man lengths as a proof of unshakeable courage on the part of the men, who thus displayed their willingness to die where they stood and not to retreat. The chains also acted as an insurance against a breakthrough by enemy cavalry. However, modern historians suggest that the Byzantines adopted the Graeco-Roman testudo military formation in which soldiers would stand shoulder-to-shoulder with shields held high and an arrangement of 10 to 20 men would be completely shielded on all sides from missile fire, each soldier providing cover for an adjoining companion.

Weaponry: The Byzantine cavalry was armed with a long sword, known as the spathion. They would also have had a light wooden lance, known as a kontarion and a bow (toxarion) with forty arrows in a quiver, hung from a saddle or from the belt. Heavy infantry, known as skoutatoi, had a short sword and a short spear. The lightly armed Byzantine troops and the archers carried a small shield, a bow hung from the shoulder across the back and a quiver of arrows. Cavalry armour consisted of a hauberk with a mail coif and a helmet with a pendant: a throat-guard lined with fabric and having a fringe and cheek piece. Infantry was similarly equipped with a hauberk, a helmet and leg armour. Light lamellar and scale armour was also used.

Tensions in Byzantine army: Khalid's strategy of withdrawing from the occupied areas and concentrating all of his troops for a decisive battle forced the Byzantines to concentrate their five armies in response. The Byzantines had for centuries avoided engaging in large-scale decisive battles, and the concentration of their forces created logistical strains for which the empire was ill-prepared.
Damascus was the closest logistical base, but Mansur, leader of Damascus, could not fully supply the massive Byzantine army that was gathered at the Yarmuk plain.