According to the will of Abu Bakr, Umar was to continue the conquest of Syria and Mesopotamia. On the northeastern borders of the Empire, in Mesopotamia, the situation was rapidly deteriorating. During Abu Bakr's era, Khalid ibn al-Walid had left Mesopotamia with half his army of 9000 soldiers to assume command in Syria, whereupon the Persians decided to take back their lost territory. The Muslim army was forced to leave the conquered areas and concentrate on the border. Umar immediately sent reinforcements to aid Muthanna ibn Haritha in Mesopotamia under the command of Abu Ubaid al-Thaqafi. At that time, a series of battles between the Persians and Arabs occurred in the region of Sawad, such as Namaraq, Kaskar and Baqusiatha, in which the Arabs managed to maintain their presence in the area. Later on, the Persians defeated Abu Ubaid in the Battle of the Bridge. It is traditionally dated to the Year 634, and was the only major Sassanian victory over the invading Muslim armies.