The Battle of Bosra was fought in 634 CE between the Rashidun Caliphate army and the Byzantine Empire over the possession of the city of Bosra, in Syria. The city was one of the Islamic forces' first significant captures and was at the time the capital of the Ghassanid Kingdom (a Byzantine vassal state). The siege took place between June and July.
Background
Caliph Abu Bakr sent his four corps under Amr ibn al-A'as, Abu Ubaidah ibn al-Jarrah, Shurahbil ibn Hasana, and Yazid ibn Abi Sufyan to capture different districts in Syria. The four initially failed due to the concentration of the Byzantine army's strength at Ajnadayn. Abu Bakr sent Khalid ibn Walid to Syria to take command of the Rashidun army there. Khalid ibn Walid reached Syria and captured several towns, after which he reached the city of Bosra in June of 634 CE. According to his instructions, Abu Ubaidah ibn al-Jarrah, who had already occupied the District of Hauran, which lay north-east of the river Yarmouk, was to remain at his position until Khalid arrived at Bosra. Abu Ubaidah ibn al-Jarrah had three corps of an army of Muslim soldiers under his command––his own, Yazid's, and Shurahbil's––but had fought no battles and captured no towns. Bosra, a large town and the capital of the Ghassanid Kingdom, contained a force of Byzantine and Christian Arabs, commanded by Roman officers.
While Khalid was in eastern Syria, Abu Ubaidah learned that he would serve under Khalid upon the latter's arrival. Upon receiving this news, he sent Shurahbil with 4,000 men to capture Bosra, the garrison of which withdrew into the fortified town as soon as the soldiers appeared in sight. This garrison consisted of 4,000 soldiers who all expected that more Islamic forces would soon arrive and that the Shurahbil detachment was only an advance guard, and remained within the walls of the fort. Shurahbil camped on the western side of the town and positioned groups of his men around the fort.
After two days, as Khalid ibn al-Walid set out on the last day of his march to Bosra, the garrison of Bosra advanced towards the Muslim army outside the city. Shurahbil and the Roman commander held talks with their forces drawn up for battle; the Muslims offered three choices: Islam, tribute, or the sword. When the Byzantines chose the sword, in the middle of the morning, the battle began.
For the first few hours, the fighting continued at a steady pace, with neither side making any headway; but soon after midday, the superior strength of the Romans turned the battle in their favor. The Romans moved forces around both of Shurahbil's flanks and the fighting increased in intensity. The temper of Shurahbil's soldiers became suicidal as the real danger of their position became evident, and they fought with ferocity to avoid encirclement, which appeared to be the Roman's goal. By early afternoon, the Roman wings had moved further forward, and the encirclement of Shurahbil's force became a virtual certainty. Then, suddenly, the combatants became aware of a powerful cavalry force galloping towards the battlefield from the northwest.
Khalid was about a mile from Bosra when he heard the sounds of battle. He immediately ordered the men to horse; as soon as the cavalry was ready, he led it towards the battle field. The Romans withdrew soon after they became aware of the cavalry's arrival. The Muslims under Shurahbil came to regard Khalid's arrival as a miracle.
The Battle
The next morning, the Byzantine garrison launched another attack. The shock of Khalid's arrival the previous day had now worn off, and seeing that the combined strength of the Muslims was about the same as their own, the Romans attacked again, hoping to defeat the Muslims before they could rest after their lengthy march.