Hazrat Khalid Bin Waleed (R) in Battle of Yarmouk

The Battle of Yarmouk was a major battle between the Muslim forces of the Rashidun Caliphate and the armies of the Eastern Roman-Byzantine Empire. The battle consisted of a series of engagements that lasted for six days in August 636, near the Yarmouk River, along what is today the border between Syria and Jordan, south-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 (PBUH) , heralding the rapid advance of Islam into the then Christian Levant.
In order to check the Muslim advance and to recover lost territory, Emperor Heraclius had sent a massive expedition to the Levant in May 636. As the Byzantine army approached, the Muslims retreated 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 also considered to be one of Khalid ibn al-Walid's (R) greatest military victories. It cemented his reputation as one of the greatest tacticians and cavalry commanders in history.