[1] From 634 to 636 AD, following the transfer of Khalid to the Byzantine front in the Levant, the hold of Arab forces on the region weakened under the pressure of Sasanian counterattacks.

After the Ridda Wars, a tribal chief of northeastern Arabia, Al-Muthanna ibn Haritha, raided the Sasanian towns in Mesopotamia (modern-day Iraq).

First, along the border between Arabia and these two great empires were numerous nomadic Arab tribes serving as a buffer between the Persians and Romans.

Second, the Persian and Roman populations were very highly taxed; Abu Bakr believed that they might be persuaded to help the Muslims, who agreed to release them from the excessive tributes.

Finally, Abu Bakr hoped that by attacking Iraq and Syria he might remove the danger from the borders of the Islamic State.

Al-Muthanna ibn Haritha went to Medina to inform Abu Bakr about his success and was appointed commander of his people, after which he began to raid deeper into Mesopotamia.

[3] To ensure victory, Abu Bakr used a volunteer army and put his best general, Khalid ibn al-Walid, in command.

After defeating the self-proclaimed prophet Musaylimah in the Battle of Yamama, Khalid was still at Al-Yamama when Abu Bakr ordered him to invade the Sasanian Empire.

On the other hand, if you do not want to embrace Islam, then as soon as you receive this message, send over the jizya and I give you my word that I will respect and honor this covenant.

[6] Expecting Khalid ibn al-Walid to come though Kazima, the Sasanian commander Hormozd Jadhuyih marched from Uballa to Kazima, where his scouts informed him that the Arab forces were instead moving towards Hufeir, a settlement only 21 miles from Uballa, in a threat to the Sasanian base, so Hormozd immediately ordered a move to Hufeir, 50 miles away.

According to Tabari, the Persian troops linked themselves together with chains as a defense against cavalry attacks and as a sign to the enemy that they were ready to die rather than to run away from the battle field in case of defeat.

The tide of the battle turned, and the assaults of the Arab forces eventually broke the Sasanian morale and the Iranian troops started fleeing towards Al-Hirah.

At this point, Khalid received a call for aid from northern Arabia at Dawmat al-Jandal, where another Muslim Arab general, Iyad ibn Ghanm, was trapped among the rebel tribes.

He accordingly marched against the border city of Firaz, where he defeated the combined forces of the Sasanian Persians, the Byzantines and Christian Arabs in December.

While Khalid was on his way to attack Qadissiyah (a key fort en route to Ctesiphon), Abu Bakr ordered him to the Roman front in Syria to assume command there.

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.

Umar immediately sent reinforcements to aid Muthanna ibn Haritha in Mesopotamia under the command of Abu Ubaid al-Thaqafi.

In 635 Yazdgerd III sought an alliance with Emperor Heraclius of the Eastern Roman Empire, marrying the latter's daughter (or, by some traditions, his granddaughter) in order to seal the arrangement.

While Heraclius prepared for a major offence in the Levant, Yazdegerd ordered the concentration of massive armies to push the Muslims out of Mesopotamia for good through a series of well-coordinated attacks on two fronts.

While Heraclius launched his offensive in May 636, Yazdegerd was unable to muster his armies in time to provide the Byzantines with Persian support.

Umar, allegedly aware of this alliance and not wanting to risk a battle with two great powers simultaneously, quickly reinforced the Muslim army at Yarmouk to engage and defeat the Byzantines.

Meanwhile, he ordered Saad to enter into peace negotiations with Yazdegerd III and invite him to convert to Islam to prevent Persian forces from taking the field.

Within three months, Saad defeated the Persian army in the Battle of al-Qādisiyyah, effectively ending Sasanian rule west of Persia proper.

After withdrawal from Ctesiphon, the Persian armies gathered at Jalawla, a place of strategic importance due to routes leading from here to Mesopotamia, Khurasan and Azerbaijan.

Thereafter, a Muslim force under Qa'qa marched in pursuit of the escaping Persians at Khaniqeen, 25 kilometres (15 mi) from Jalawla on the road to Iran, still under the command of Mihran.