Khalid bin Al-Waleed was one of the greatest generals in history, and one of the greatest heroes of history. Besides him, Genghis Khan was the only other general to remain undefeated in his entire military life. Khalid was sent to the Persian Empire with an army consisting of 18,000 volunteers to conquer the richest province of the Persian empire, Euphrates region of lower Mesopotamia, (present day Iraq). Khalid entered lower Mesopotamia with this force.?He won quick victories in four consecutive battles: the Battle of Chains, fought in April 633; the Battle of River, fought in the third week of April 633; the Battle of Walaja, fought in May 633 (where he successfully used a double envelopment manoeuvre), and Battle of Ullais, fought in the mid-May 633.?In the last week of May 633, al-Hira, the regional capital city of lower Mesopotamia, fell to Khalid. The inhabitants were given peace on the terms of annual payment of jizya (tribute) and agreed to provide intelligence for Muslims.?After resting his armies, in June 633, Khalid laid siege to Anbar which despite fierce resistance fell in July 633 as a result of the siege imposed on the town.?Khalid then moved towards the south, and captured Ein ul Tamr in the last week of July, 633.