The Umayyad & Abbasid Caliphates

Muhammad died in 632 CE without designating a clear successor. Within a decade, Arab armies had shattered two of the most powerful states on earth. Within a century, the new Islamic empire stretched from the Iberian Peninsula to the Indus Valley — a distance greater than the width of the continental United States.

Nothing in seventh-century geopolitics predicted this. The Arab tribes of the peninsula had no tradition of sustained empire-building. They were fractious, decentralized, constantly at war with each other. What changed was faith, organization, and opportunity. The Byzantine and Sasanian Persian empires had just finished an exhausting twenty-six-year war that left both sides depleted, their frontier populations resentful of heavy taxation and religious persecution. When Arab armies arrived, they often found populations that were at least willing to wait and see, if not actively relieved to see the old rulers gone.

Speed mattered too. Arab cavalry moved faster than the lumbering armies of settled empires. The first great general of Islamic expansion, Khalid ibn al-Walid, fought several pitched battles in Syria and Iraq in rapid succession, never giving his opponents time to regroup. He reportedly fought in over a hundred battles without a single defeat. Whether that number is accurate hardly matters. The legend reflected a real tactical reality: these armies were fast, flexible, and highly motivated.