One of the outstanding military geniuses of Islam. Khalid was launched on a  brilliant military conquest for Islam which continued under the rule of the first two caliphs, Abu-Bakr (A.D. 632-634) and `Umar ibn-al-Khattab (634-644). it is also recorded that he who ordered the repayment of the Christian tribute was the commander-in-chief, Abu-'Ubaydah.