In 636, on this day the combined armies of the Byzantine Empire and Ghassanid Kingdom defeated Muslim forces of the Rashidun Caliphate in a major battle fought near the Yarmouk River, south-east of the Sea of Galilee. Correctly anticipating that Khalid ibn al-Walid was a fine cavalry commander who would make imaginative use of his scarce resources, the Armenian Commander-in-Chief Prince Vahan successfully managed to deal with the mounted force, countering with his own Byzantine horse and also using the sizable advantage of his armies effectively.
Famous Byzantine Victory at the Battle of YarmoukThis famous Byzantine victory was a personal triumph for the Emperor Heraclius who had recognized the rapid advance of Islam into the Christian Levan and sent a massive expedition in order to check the Muslim advance and to recover lost territory. It was a huge risk for Heraclius who took the bold decision to draw military resources away from the defence of the Empire's main granary in Egypt (this commitment gave Vahan a decisive 10:1 numerical advantage). The basis of the Emperor's calculations was that the Arab armies were still very dependant on being successful in battle in order to reward auxiliary followers with levies. He correctly assumed that ongoing defeats would eventually lead to a collapse of the war effort and the Arabs withdrawing from the region. But of course this required the triumph at Yarmouk to be followed up on with more Byzantine and Persian victories.