This defeat left Syria vulnerable to the Muslim invaders. Khalid decided to capture Damascus, the Byzantine stronghold. At Damascus, Thomas, son-in-law of Emperor Heraclius, was in charge. Having received intelligence of Khalid's march towards Damascus, he prepared for its defence, writing to Emperor Heraclius in Emesa for reinforcements. Moreover, Thomas, in order to get more time for preparation of a siege, sent armies to delay or, if possible, halt Khalid's march to Damascus. One of these armies was defeated at the Battle of Yaqusa in mid-August near Lake Tiberias, 145 kilometres (90 mi) from Damascus. Another was defeated in the Battle of Maraj as Saffer on 19 August. These engagements had the desired effect, delaying Khalid long enough to prepare for a siege. However, by the time Heraclius' reinforcements had reached the city Khalid had begun his siege, having reached Damascus on 20 August. To isolate the city from the rest of the region Khalid placed detachments south on the road to Palestine and in the north at the Damascus-Emesa route, and several other smaller detachments on routes towards Damascus. Heraclius' reinforcements were intercepted and routed at the Battle of Sanita-al-Uqab, 30 kilometres (20 mi) from Damascus. Khalid's forces withstood three Roman sallies that tried to break the siege. Khalid finally attacked and conquered Damascus on 18 September after 30 days, although, according to some sources, the siege had in fact lasted for four or six months. Heraclius, having received the news of the fall of Damascus, left for Antioch from Emesa. The citizens were granted peace on the promise of annual tribute and the Byzantine army was given three days to go as far as they could. After three days, Khalid took a cavalry force, caught up to the Romans using an unknown shortcut, and attacked them at the Battle of Maraj-al-Debaj, north of Damascus.