Umar's decade as caliph (634-644) transformed a young polity in Arabia into an empire. Under his command Muslim armies took Syria and Palestine from the Byzantines, seizing Damascus and Jerusalem; overran Egypt; and shattered the Sasanian Persian empire at Qadisiyya and Nihavand, opening Iraq and Iran. Few conquests in history moved so far so fast, and later Islamic memory credits his generalship and, above all, his administration.
For Umar is remembered less as a warrior than as the organizer of the state. Tradition ascribes to him the diwan, the register through which conquered revenue was distributed; the retention of existing Byzantine and Persian bureaucracies; the founding of garrison cities such as Kufa and Basra; and the establishment of the Hijri calendar, dating the Muslim era from the migration to Medina. He is said to have taken the title Amir al-Mu'minin, Commander of the Faithful, thereafter borne by the caliphs.
The treatment of conquered non-Muslims under his rule became a lasting reference point. The so-called Pact of Umar, a set of conditions governing the protected dhimmi communities, is attributed to him, though scholars judge much of it to have taken shape later and been projected back onto his authoritative name.