The Battle of Yarmouk was a major battle between the army of the Eastern Roman Empire (Byzantine Empire) and the Muslim Arab forces of the Rashidun Caliphate.
The battle consisted of a series of engagements that may have lasted for six days in August 636, near the Yarmouk River, along what today are the borders of Syria–Jordan and Syria–Israel, east of the Sea of Galilee. The result of the battle was a complete Muslim victory which ended Byzantine rule in Syria. The Battle of Yarmouk is regarded as one of the most decisive battles in military history, and it marked the first great wave of Islamic conquests after the death of Muhammad, heralding the rapid advance of Islam into the then Christian Levant.
In order to check the Arab advance and to recover lost territory, Emperor Heraclius had sent a ‘massive’ expedition to the Levant in May 636. (Reports that the Byzantine army numbered 100,000+ are clearly an exaggeration – the combined field force was probably more like 30,000 – 40,000.) (Arab strength was probably about 20,000 – 25,000.) As the Byzantine army approached, the Arabs tactically withdrew from Syria and regrouped all their forces at the Yarmouk plains close to Arabia where, after being reinforced, they defeated the numerically superior Byzantine army. The battle is considered to be one of Khalid ibn al-Walid’s greatest military victories. It cemented his reputation as one of the greatest tacticians and cavalry commanders in history.