He is famous for his military success, as he commanded the forces of Muhammad and of his immediate successors Abu Bakr and Umar ibn al-Khattab. He was undefeated in over a hundred battles against the numerically superior forces of the Byzantine Roman Empire, Sassanid Persian Empire, and their allies. Therefore he is regarded as one of the finest military commanders in history. His greatest strategic achievements were his quick conquest of the Persian Empire and conquest of Roman Syria within three years from 633 to 636. His greatest tactical achievements were his successful double envelopment maneuver at Walaja and his victories at Ullais and Yarmouk.