Battle of Yarmuk (Yarmuk)

The Battle of Yarmuk, fought in 636 AD, was the turning point in Arab history that sealed the decline of the Rashidun Caliphate right from its very beginnings. Fought between Arab general Khalid ibn al Walid and the Byzantine Emperor Heraclius, it showed that Byzantine arms would not be tarnished by the newfound tactics of the Arabs.

The Plan
Harnessing the zeal of Arabia's new religion, Caliph Abu Bakr devised a plan of action to expand the tiny Rashidun Caliphate beyond his peoples wildest dreams. A profound mathematician, Abu Bakr had worked out the exact timings involved to knock out his two most immediate enemies in one go. Both Persia and Byzantium were large but essentially weak nations, with a hatred inherited from years of rivalry and warfare. There was no one else of any import in the area (save the Khazars, who had admittedly slipped through Abu Bakr's calculations) and consequently he felt it would be easiest first to eliminate the Byzantines, the stronger of Arabia's future opponents, and then to use what was left of this force to conquer Persia. Having shared his plans with Khalid ibn al Walid, his strongest commander, he was ready to initiate his invasion plans by the opening months of 636AD.