Umar ibn al-Khattab (c. 584 – 644) served as the second Rashidun Caliph, ruling from August 634 until his assassination in 644. Initially a strong opponent of the Islamic prophet Muhammad, his distant kinsman, Umar converted in 616 and became a devoted companion who earned the title al-Fārūq ("the Distinguisher") for his sound judgment. Succeeding Abu Bakr, Umar's reign saw the Islamic Caliphate expand at an unprecedented rate, conquering the entire Sasanian Empire and over two-thirds of the Byzantine Empire, including Persia within two years (642–644).
He is also credited with lifting the Christian ban on Jews entering Jerusalem and introducing the Hijri Calendar. Widely regarded as one of the most powerful and influential Muslim caliphs in history, Umar is revered in Sunni Islam as a just ruler and paragon of virtues, although he is viewed negatively in Twelver Shia tradition.