To check the Arab advance and to recover lost territory, Emperor Heraclius had sent a massive expedition to the Levant in May 636. As the Byzantine army approached, the Arabs tactically withdrew from Syria and regrouped all their forces at the Yarmouk plains close to the Arabian Peninsula, where they were reinforced and defeated the numerically superior Byzantine army. The battle is Khalid ibn al-Walid's greatest military victory and cemented his reputation as one of the greatest tacticians and cavalry commanders in history.