The Mobile Guard (Arabic? , Tulay'a mutaharikkah or , "Jaish al-Za?f") was an elite light cavalry regiment of Rashidun army during the Muslim conquest of Syria, under the command of Khalid ibn Walid. This force was earmarked as a cavalry reserve for use in battle as required.
After the decisive victory at the Battle of Ajnadayn in 634 CE, Khalid, from his army of Iraq, which after Ajnadayn numbered about 8000 men, organised a force of 4000 horsemen, which the early historians refer to as The Army of Sharpeners. Khalid kept this force under his personal command.
The first recorded use of this mounted force was during the Siege of Damascus (634). The best use of this lightly armed fast moving cavalry was revealed during the Battle of Yarmuk (636 AD) 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 disengage, 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 rout opposing armies with its greater mobility that gave it an upper hand against any Byzantine army. One of the victories of the mobile guard was at Battle of Hazir in 637 CE 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 the surrender of Antioch.
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 CE) until Khalid was dismissed from army by Caliph Umar after the completion of the conquest of the Levant.
With the dismissal of Khalid this powerful cavalry regiment was dismantled. One of its brilliant commanders Qa'qa ibn Amr had been sent to the Persian front in 637 CE along with reinforcements for the Battle of al-Q?disiyyah, in which he played an important part. A part of it was later sent to the Persian front as reinforcements for the Muslim conquest of Persia. Many of its members died in the plague during 639-640 CE which killed approximately 25,000 Muslims in Syria. This included many sub-commanders of the mobile guard like Zirrar ibn Azwar, those who survived accompanied the army under the command of Amr ibn al-'As to conquer Egypt.