Khalid immediately set out for Syria from Al-Hirah, in Iraq, in early June, taking with him half his army, about 8000 strong. There were two routes towards Syria from Iraq: one was via Daumat-ul-Jandal, and the other was through Mesopotamia, passing through Raqqa. The Muslim armies in Syria were in need of urgent reinforcement, so Khalid avoided the conventional route to Syria via Daumat ul Jandal, as it was the longer route, and would take weeks to reach Syria. Khalid avoided the Mesopotamian route because of the presence of Roman garrisons there and in Northern Syria. To engage them at a time when Muslim armies were being outflanked in Syria was not a wise idea. Khalid selected a shorter route to Syria, an unconventional route passing through the Syrian Desert. It is recorded that his soldiers marched for two days without a single drop of water, before reaching a predetermined water source at an oasis. Khalid thus entered Northern Syria and caught the Byzantines on their right flank. According to modern historians, this ingenious strategic maneuver unhinged the Byzantine defences in Syria.