After being deposed in 610, Phocas was succeeded by Heraclius, who led the Byzantines in a war of reconquest, successfully regaining territory lost to the Sasanians.

The Göktürks, attacking the north of Persia with a massive army during peace proceedings, were ordered by Heraclius to retreat after the signing of the pact with Kavadh.

After a few short months of reign, Kavadh II suddenly died of the plague; the ensuing power vacuum quickly led to a civil war.

Ardashir III (c. 621–630), son of Kavadh II, was raised to the throne at age seven, but was killed 18 months later by his general, Shahrbaraz, who then declared himself ruler.

She was made empress regnant on the understanding that she would vacate the throne upon Yazdegerd III (632–651), the son of Shahriyar and Grandson of Khosrow II, attaining majority.

However, Boran was largely unsuccessful in restoring the power to the central authority, which had been weakened considerably by civil wars, and either resigned or was murdered soon after ascending to the throne.

After the death of Muhammad, Abu Bakr established control over Arabia through the Ridda Wars and then launched campaigns against the remaining territories of Syria and Palestine.

To make victory certain, Abu Bakr decided that the invading army would consist entirely of volunteers and would be commanded by his best general, Khalid ibn al-Walid.

Khalid then moved towards the south and conquered the city of Ein ul Tamr after the Battle of Ayn al-Tamr in the final week of July 633.

Al-Muthanna ibn Haritha, who was now commander-in-chief of the Muslim forces in Iraq, pulled his troops back from all outposts and evacuated Al-Hirah.

The reinforcements reached Iraq in October 634, and Abu Ubaid assumed the command of the army and defeated the Sassanids at the Battle of Namaraq near modern-day Kufa.

This "tit-for-tat" struggle continued until emperor Yazdegerd III consolidated his power and sought an alliance with Heraclius in 635 in an effort to prepare for a massive counterattack.

Simultaneously, Umar engaged Yazdegerd III, ordering Sa'd ibn Abi Waqqas to enter in peace negotiations with him by inviting him to convert to Islam.

Receiving news of preparations for a massive counterattack, Umar ordered Muthana to abandon Iraq and retreat to the edge of the Arabian Desert.

Due to a shortage of manpower, Umar decided to lift the ban on the ex-apostate tribes of Arabia from participating in state affairs.

After a decisive victory against the Byzantine army at the Yarmouk, Umar sent immediate orders to Abu Ubaidah to send a contingent of veterans to Iraq.

These figures come from studying the logistical capabilities of the combatants, the sustainability of their respective bases of operations, and the overall manpower constraints affecting the Sassanids and Muslims.

[citation needed] The Persian army reached Qadisiyyah in July 636 and established their highly fortified camps on the eastern bank of the Ateeq river.

Rostam himself was stationed at an elevated seat, shaded by a canopy, near the west bank of the river and behind the right centre, where he enjoyed a wide view of the battlefield.

After establishing camp, organizing defences, and securing river heads, Sa'd sent parties inside Suwad to conduct raids.

The army was formed on a tribal and clan basis so that every man fought next to well-known comrades and so that tribes were held accountable for any weakness.

Umar ordered Sa'd to send emissaries to Yazdegerd III and the general of the Sasanian army, Rostam Farrokhzad, inviting them to convert to Islam.

An-Numan ibn Muqarrin led the Muslim emissary to Ctesiphon and met Sasanian Emperor Yazdgerd III, but the mission failed.

During one meeting, Yazdgerd III, intent on humiliating the Arabs, ordered his servants to place a basket full of earth on the head of Asim ibn 'Amr al-Tamimi, a member of the emissary.

As there were no elephants in the Sassanid fighting force that day, Sa'd sought to exploit this opportunity to gain any breakthrough if possible, so he ordered a general attack.

Through the gaps that had appeared in the foe's ranks because of the Sassanid advance, Rostam sent a cavalry regiment to capture the old palace where Sa'ad was stationed.

Sa'd determined that there was only one way to win the battle; to destroy the Sassanid elephant corps that was causing great havoc among the Muslim ranks.

This account has been dismissed as unlikely due to several problems with the story, including the presence of suspicious literary devices and general inconsistencies in the narrative.

From this battle, the Arab Muslims gained a large number of spoils, including the famed jewel-encrusted royal standard, called the Derafsh-e-Kāveyān (the 'flag of Kāveh').

As long as the Sassanids held their capital Ctesiphon, there was always the danger that at some suitable moment they would attempt to recover what they had lost and drive away the Muslims from Mesopotamia.