After the decisive victory at the Battle of Ajnadayn in 634 A.D, Khalid, from his army of Iraq, which after Ajnadayn numbered about 8,000 men, organised a force of 4,000 horsemen, which the early historians refer to as The Army of Movement. For want of a better translation, it shall be called the Mobile Guard. Khalid kept this force under his personal command.
The first recorded use of this mounted force was during the siege of Damascus. The best use of this lightly armed fast moving cavalry was revealed during the Battle of Yarmouk (636 A.D) in which Khalid ibn Walid, knowing the importance and ability of his cavalry, used them to turn the course of events at every critical instance of the battle. With their ability to engage and dis-engage, and turn back and attack again from the flank or rear, the Mobile Guard inflicted a shattering defeat of the Byzantine army. This strong mobile striking force was often used in later years as an advance guard. It could route opposing armies with its greater ability of mobility that give it an upper hand against any Byzantine army and cavalry. One of the victories of the mobile guard was at Battle of Hazir in 637 A.D under the command of Khalid, in which not a single Byzantine soldier survived.With this mobile striking force the Muslims easily conquered Syria with few casualties, including the Muslim victory at Battle of Iron bridge which followed by the surrender of Antioch city.
Khalid ibn Walid had organized a military staff – a simple beginning of what later in military history would emerge as the General Staff. He had collected from all the regions in which he had fought-Arabia, Iraq, Syria and Palestine a small group of keen and intelligent men who acted as his 'staff officers', mainly functioning as an intelligence staff. They would collect information, organize the dispatch and questioning of agents, and keep Khalid up-to-date with the latest military situation. This was a personal staff rather than the staff of an army headquarters; wherever Khalid went, this staff went with him, and was part of the mobile guard. The Mobile guard remained under the personal command of Khalid ibn Walid for about four years (634 - 638 A.D ) until Khalid was dismissed from army by Caliph Umar after the completion of the conquest of Levant.
With the dismissal of Khalid this powerful cavalry regiment was dismantled. One of its brilliant commander Qa'qa ibn Amr was already sent to Persian front in 637 A.D along with reinforcement for the Battle of al-Qādisiyyah, he played a heroic role in the battle. A part of it was later sent to Persian front as a reinforcement for the Islamic conquest of Persia. Many of its eminent riders died in the plague during 639 - 640 A.D which approximately killed 25,000 Muslims in Syria, including many of sub-commanders of Mobile guard like Zirrar ibn Azwar, those who survived accompanied the army under the command of Amr ibn al-'As to conquer Egypt. The Mobile Guard was undoubtedly the finest body of men in the Rashidun army-a corps d'elite.