5 – Battle of Yarmouk (AD 636)
The Battle of Yarmouk is probably not as well-known as it should be given its enormous historical significance. It pitted Byzantine forces led by Emperor Heraclius against the Rashidun Caliphate. It was a six day battle which began on 15 August AD 636 near the River Yarmouk along the modern day Syria-Jordan and Syria-Israel borders.
Heraclius had enjoyed a succession of victories against the rival Sassanid Empire but a quarter of a century of constant warfare wore down both sides. By AD 634, Arab forces had begun to invade the Byzantine east with a series of small raids. Impressive victories at Ajnadayn and Pella caused the influence of Christianity in the Levant to weaken and when Damascus fell to the Arab forces in AD 635, it was clear to Heraclius that he needed to check their advance before they became unstoppable.
He wanted to quickly regain Damascus and raised an army to march on the city. The Arab forces abandoned their Syrian raids and retreated to the River Yarmouk. The armies actually met in May AD 636 but the Byzantines didn’t engage until the middle of August. This was a huge mistake as it gave the Arabs time to reinforce, close off the Deraa Gap and scout enemy positions. The closure of the Deraa Gap was to prove critical as it prevented the Byzantine army from safely retreating.
Even though the Byzantines held an estimated 4:1 advantage in manpower, the tactical ability of Khalid ibn al-Walid was a major factor in the Arab victory. The Arab forces managed to gradually gain the upper hand and on the sixth day of the battle, their cavalry routed their Byzantine counterparts and this enabled them to attack the enemy rear which led to a retreat. However, the Byzantines had nowhere to run and were slaughtered by the well-trained Arab army. Casualty estimates vary but it is believed that the Byzantines lost up to 70,000 men in the Battle of Yarmouk.
The defeat resulted in the Byzantines permanently losing Syria, Palestine, Egypt and even parts of Mesopotamia. It also led to the quick collapse of the Sassanid Empire which was easy prey for the marauding Arab army. In addition, it led to the first great wave of Islamic conquests after Muhammad’s death and resulted in the rapid advance of Islam into previously Christian areas.