 With the collapse of the rebellion, and Arabia united under the central authority of the caliph at Medina, Abu Bakr decided to expand his empire. It is unclear what his intentions were, whether it was a full scale expansion plan or pre-emptive attacks to secure more territory to create a buffer zone between the Islamic state and the powerful Sassanid and Byzantine empires. Khalid was sent to the Persian Empire with an army consisting of 18,000 volunteers to conquer the richest province of the Persian empire, Euphrates region of lower Mesopotamia, (present day Iraq). Khalid entered lower Mesopotamia with this force. It is recorded that before engaging the Sassanid forces Khalid wrote a letter to taunt his opponents to surrender:
