After the defeat of Buwayb, the Persian chiefs and nobles buried their differences and mobilized their forces to serve their country even in the face of death. Rustam and Fayrouz (prime minister of the Persian Empire) were the pillars of the State, but a violent friction raged between them. Now both of them were persuaded to shake hands in the interest of the Persian Empire. The coronation of Yezdgird also infused new life into those who were disheartened because of the adverse state of affairs in every field. The provinces and cities under the possession of Muslim officers began to show signs of unrest and rebellion. The Persian camps were packed with soldiers and the Persian forts and military outposts were fortified and strengthened. Many other regions under Muslim control broke into revolt and rose in support of the Persians.

The Caliph decides to lead the Muslim army

'Umar came to know of these fresh developments in the month of Thul-Qi'dah in Al-Madeenah. He issued prompt orders for Muthanna bin Harithah together with all the troops to fall back towards the frontiers of Arabia. He summoned the tribes of Rabee'ah and Mudhar that were scattered throughout Iraq strengthened his forces and vacated the threatened areas to gather close to the frontiers of Arabia. He also issued orders to the governors to collect and send warriors to fight in the way of Allah. As the season for the pilgrimage had arrived, 'Umar set off to Makkah.

On returning from the Hajj, he found Arab tribes pouring into Al-Madeenah from all sides. The suburbs of Al-Madeenah were now teeming with groups of warriors. He entrusted the divisional command of the vanguard to Talhah and that of the right wing to Az-Zubayr while 'Abdur-Rahman bin 'Awf was appointed to the command of the left wing of the army. When the army was drawn up, he put 'Ali in charge of the Caliphate, left Al-Madeenah, and advanced towards Persia. At Sirar, the first halt was ordered.

The fact that the Caliph himself was leading the army filled them with unbounded confidence and enthusiasm. However, 'Uthman bin 'Affan called on the Caliph and said that it was not expedient that he should go personally into the battlefield. Following this advice, 'Umar set up a general council of war at Sirar and invited the opinion of everyone present. Everyone unanimously exclaimed that the expedition could not terminate successfully unless he led it himself.

Thereupon, 'Abdur-Rahman bin 'Awf said: “I disapprove of such a suggestion. The Caliph's presence on the battlefield is too risky. In case a commander is killed in action, the Caliph can do what is necessary to keep the situation under control; but if Allah forbid, the Caliph himself is eliminated, it would be extremely difficult to manage the affairs.” 'Ali was also called from Al-Madeenah to take part in this crucial deliberation. He and the other Companions lent support to 'Abdur-Rahman bin  'Awf’s opinion.

The Caliph agreed not to lead the campaign. After a long discussion about who would take command of the Muslim army at this juncture, Sa'd bin Abu Waqqas was named. The entire council, including 'Umar agreed.

The Battle of Qadisiyah

Sa’d was at Siraf when he received a fresh order from the Caliph directing him to proceed towards Qadisiyah. The order further enjoined him to arrange himself and his troops in such a manner, so as to have the plains of Persia in front and the hills of Arabia in the rear. In this way, he might advance as far as he chose in case of victory, and take refuge by retreating to the hills in case of defeat.

News began to pour into the Persian capital that the Arabian army was encamped in Qadisiyah and they had ravaged the surrounding areas of the Euphrates. The Persian leader, Rustom, marched up to Sabat where he was joined by forces from almost every part of the country in such great numbers that, in a short time, the Persian army numbered nearly 180,000 men. It was not only a well-equipped army, but also showed a rage and enthusiasm against the Islamic forces.

Armed with war equipment and weapons on such a massive scale, Rustam marched from Sabat and camped at Kutha. Now the distance between the Persian and the Muslim armies was much closer. Small raiding squads would come out from both sides to pounce on the other's provisions and other things of necessity.

Rustam ordered preparations for a decisive battle. He ordered a bridge to be constructed over a canal that separated the armies, and it was completed within a short period. Rustam then enquired from his counterpart as to who should cross the bridge, and Sa’d invited him to cross. Thus, the large and strong Persian army moved across the bridge and battle lines were drawn up. Rustam launched an all-out assault on the Muslim troops, and by way of a war strategy, combat elephants were set off to attack the Muslim ranks. The Bujaylah tribe obstructed them at the cost of heavy casualties. Sa’d who was watching the battle scene very minutely, reinforced the Bujailah with Banu Asad who showed utmost manliness in the assigned duty. However, when they too showed signs of reverses, the warriors of Banu Kindah took the field and made such a heavy charge that the Persians were forced to retreat. In view of constant retreat and repulses, Rustam ordered a joint attack. Sa’d cried the Takbeer (Allahu Akbar – Allah is the Greatest) at the top of his voice and the entire Muslim army joining his Takbeer, charged against the Persian troops. It looked as if two oceans or mountains had collided with each other. When the rival forces were in the thick of battle, the Persian elephants began to cause heavy casualties on the Muslim side. Sa’d immediately ordered the archers to shoot arrows at the elephants and their riders. 'Aasim charged at the elephants with his lance, followed by others who inflicted deep wounds on the elephants' trunks with their spears and swords. As a result, the elephants retreated leaving the Muslim swordsmen to display their bravery. After a daylong battle, night intervened to stop it until the next day.

After fierce fighting that lasted for three days, all the tribes rose as one man to charge forcefully at the enemy. When the horsemen of Al-Qa'qaa' reached near Rustom, he got down from his throne and began to fight. However, on being wounded he took to his heels. But Hilal bin Ulafah chased him and hit him so powerfully with his spear that his hip was broken and he fell down in a nearby canal. Hilal dismounted from his horse at once, pulled him out by his legs and put him to death. Following this, Hilal called out at the top of his voice standing on Rustam's throne: "By Allah, I have killed Rustam." Having heard this announcement, the Muslim troops cried Allahu Akbar (Allah is the Most Great) and the Persian soldiers were left shocked and astonished. They fled the battlefield. Out of  30,000 Persian cavaliers, only 30 saved their lives. About 6000 Muslims were honored with martyrdom.

 Conquest of the Persian capital

After their flight from Qadisiyah, the Persians quartered themselves at Babylon. A number of renowned generals prepared themselves for battle again. The fugitives of the battle of Qadisiyah were also collected and encouraged to avenge their defeat. Sa’d stayed in Qaadisiyah for about two months after the Muslim victory. On receiving fresh orders from the Caliphate, he marched to Mada'in leaving his family in Qadisiyah. With the news of the arrival of Sa’d the Persian generals left Babylon and moved to Mada'in, Ahwaz and Nihawand destroying the bridges on the way and making the Tigris and its canals impossible to cross. When Sa’d arrived at the bank of the Tigris he found neither bridge nor boats. The next day Sa’d, may Allah be pleased with him, mounted on his horse and said after getting his troops ready: "Who among you is brave enough to promise to save me from an enemy onslaught while I cross the river?" 'Aasim bin 'Amr came forward and offered his services.

 He then charged right into the surging water of the Tigris. Others also followed suit and rushed their horses into the river. The river was deep and fast moving but the turbulent conditions could not affect the resolute and undaunted spirits of the Muslim army. The waves slammed furiously against the sides of the horses, but the horsemen steered their course calmly and in perfect order. When the cavalry was halfway across the river, the Persian archers began to shoot arrows at the Muslim troops but in vain. The Muslim fighters crossed the river by force and put the opposing force to death.

With the news of the crossing of the river by the Muslims, Yezdgird took flight from Mada'in. The Muslim troops began to enter the city from different directions. Sa’d stepped in the White Palace (royal palace) reciting the verses (which mean):

"How much they left behind of gardens and springs. And crops and noble sites. And comfort wherein they were amused. Thus! And we caused to inherit it another people. ” [Quran 44:25-28]

He offered eight Rakahs (units) of victory prayer. In the palace of Kisra (Chosroes), a pulpit was set up in place of the royal throne and the Friday prayer was performed there. This was the first Friday prayer that was performed in the Persian capital.

The fall of Mada'in, the Persian capital, was followed by that of Ahwaz, Nahawand and Hamadan but the latter rose in revolt after only a few days. Being fed up with the continual revolts of the Persian regions, 'Umar later ordered a general attack which resulted in victory. Thus, the Muslims captured all the Persian land and the empire of Magians became extinct.

To be concluded

The caliphate of 'Umar -I

The caliphate of 'Umar -III