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.

Heraclius now ordered Artabun, a noted general, to gather troops in Ajnadain. Artabun kept a huge army under his direct command and two other detachments in Ramlah and Jerusalem. The opponents of Islam were well-equipped and huge in number, awaiting the arrival of the Muslim force. 'Amr bin Al-'Aas  marched to Ajnadain to face Artabun while he dispatched Alqamah bin Hakeem Firasi and Masroor bin Al-Akki to Jerusalem and Abu 'Ayyoob Al-Maliki to Ramlah with the permission of Abu 'Ubaydah . A fierce battle was fought in Ajnadain. It was a conflict similar to Yarmook. Artabun could not muster courage to face 'Amr bin Al-'Aas and fled to Jerusalem and the city fell to the Muslim forces.

'Umar  convened a meeting of all the distinguished Companions and consulted them. 'Uthman  declared that the Christians had been struck with terror and had lost heart and that if the Caliph were to reject their request, they would be still more humiliated, and consider that the Muslims regarded them with utter contempt, they would lay down their arms unconditionally. 'Ali  however, dissented from this view and gave the contrary opinion; and 'Umar shared the same opinion.

On this historic mission to Jerusalem a bag full of parched barley meal, a camel, a slave, and a wooden cup were all the belongings of 'Umar  the Chief of the Muslims, when he left Al-Madeenah, the headquarters of Islam. Leaving 'Uthman  in charge of Al-Madeenah, he set out on the journey noted for its strain and stress.

It was a unique scenario of Islamic equality and human dignity, that at times, the Caliph sat on the camel and the slave walked along holding the rein of the camel and at other times, vice versa. It was the journey of a magnificent and powerful Islamic ruler whose cavalry had already trampled down palaces and crowns and thrones under the hooves of its horses. It was Rajab 16 AH (After Hijrah), when Mada'in and Antakiyah (Antioch) had been conquered.

The commanders of the Muslim force at Damascus and Jerusalem had already been informed about the movement of the Caliph of Islam. Thus Yazeed bin Abu Sufyan, Abu 'Ubaydah bin Al-Jarrah and Khalid bin Al-Waleed  received the Caliph of Islam with exemplary honor. However, when 'Umar  saw them arrayed in brilliant dresses and imposing appearance, he flew into a fit of rage at the sight of them and remarked: "Within the short span of two years have you fallen into Persian habits?" However, when the officers explained that they had their weapons beneath their luxurious dresses and they had not lost their Islamic character, the Caliph gained peace of heart.

The Caliph stayed for a long while at Jabiah, where some of the nobles of the city proceeded to see him and the treaty was drawn up there. The elite of the Companions like Khalid bin Al-Waleed, 'Amr bin Al-'Aas, ‘Abdur-Rahman bin ‘Awf and Mu'awiyah  subscribed to it.

The Conquest of Egypt

During 'Umar's stay in Jerusalem 'Amr bin Al-'Aas had obtained his consent for launching an attack on Egypt. 'Amr  marched to Egypt at the head of 4000 troops. In his dispatch from Al-Madeenah, the Caliph of Islam put before Muqawqis, the king of Egypt three conditions: accept Islam or pay Jizyah (poll protection tax as a sign of their surrender to Muslims) or prepare for battle. The Roman general Artabun along with his entire army was in Egypt at that time. First Artabun moved forward and then fled the battlefield after experiencing a decisive defeat.

Afterwards the Muslim army advanced further and laid siege around 'Ayn ash-Shams and from there dispatched two squadrons to besiege Farama and Alexandria. Both the cities fell to the Muslim troops. 'Amr bin Al-'Aas then sent Az-Zubayr bin Al-Awwam  to Fustat as a commander; he conquered the fortified citadel after a heavy encounter. 'Amr bin Al-'Aas attacked Alexandria which fell after a siege of three months.

The area of conquests during the Caliphate of 'Umar  is said to have spread over 2,2,500,000 square miles. This was the result of victories won by people who were once considered “small and wretched” against the mighty empires of Persia and Rome. The conquests of the Caliph 'Umar include Persia Iraq Jazeerah, Khurasan, Baluchistan Syria Palestine Egypt and Armenia. The provinces carved out by 'Umar  himself in 22 AH, were Makkah, Al-Madeenah, Syria, Jazirah, Basrah, Kufah, Egypt, Palestine, Khurasan, Azerbaijan and Persia. Some of them were equal to two provinces in area, with two centers of power and separate governors and their administrations.

Firsts accomplished by 'Umar   

'Umar  invented and enforced many things in the sphere of financial, political, administrative and social spheres, which are accomplishments first instituted by him. Some of them are mentioned below:

· He set up a formal Bayt-ul-Mal or public treasury and introduced the Hijri calendar

· He adopted the appellation of Ameer Al-Mu'mineen (Commander of the Faithful)

·       He established a regular department for the military; a separate department for financial affairs, and fixed the salaries for men in voluntary services.

·He also introduced the practice of measuring the land and keeping its record, adopted a census system, he had canals dug and populated cities like Koofah, Basrah, Jeezah, Fustat (Cairo) and delineated provinces out of the occupied territories.

· He was first to allow traders of rival countries to enter Muslim territories for the purpose of business.

· He was also the first to make use of the whip for corporal punishment and set up a prison and police department.

· He introduced a system of collecting direct information concerning states and conditions of the masses, he established a secret intelligence service.

·He had wells bored, built houses and fixed a daily payment for the destitute among the Christians and the Jews.

Martyrdom of 'Umar Al-Farooq

The next day when the people assembled in the mosque to perform the morning prayer, Fayrouz came into the mosque armed with a poisonous dagger. As the ranks of the congregation were put straight and in order, and 'Umar  came up and took his position at the head of the ranks to lead the prayer, Fayrouz suddenly rushed from the first rank and struck 'Umar six consecutive blows, one of which fell below his navel.