The purposes of the conquest:

1. Though the wars of apostasy had come to an end, Abu Bakr wanted to be sure that the vanquished ¹ tribes would not think of retaliation. The best way to ensure this was to direct their thoughts towards conquests outside the Arabian peninsula  ² .
    

2. Along the borders between Arabia and 'Iraq there were numerous Arab tribes leading a nomad life and forming a sort of buffer ³  state between the Persians and the Romans. Abu Bakr hoped that these tribesmight accept Islam and help their brethren ⁴  in spreading it.
    

3. Some of these northern tribes had replaced their nomad  ⁵  society with a rural  ⁶ one. The Persian taxation laws were arbitrary  ⁸  and oppressive; Abu Bakr believed that they might be persuaded to help the Muslims, who sought to release them from injustice.
    

4. Arabia was surrounded by two gigantic  ⁹  empires, and it was unsafe to remain passive with these two for midable powers on its borders. Abu Bakr hoped that by attacking `Iraq and Syria he might remove ¹⁰ the danger from his borders.
    

5. After the heavy defeat which the Romans inflicted on the Persians, the latter were in a state of confusion. In four years, nine kings ascended  ¹¹  the throne. By starting with Iraq, Abu Bakr hoped that the conquest of Persia might be possible and even easy.
    

6. Iraq and Syria were rich in resources ¹²  and had moderate climates.' Iraq was called "the paradise of the earth", and this worked like a charm on the minds of the Bedouins, whose pre-Islamic was merely a series of raids ¹³ .
    

7. We have already heard something of the Bahraini leader Muthanna, who helped Al-Hadhrami to subdue his own native apostates. Muthanna was not satisfied with what he had done, but marched northwards along the coast of the Gulf until he reached the borders of `Iraq. In order to invade the Persian Empire he needed Abu Bakr's consent. So, he travelled to Madina where, after taking his counsellors' advice, particularly that of Khalid bin AI-Waleed, Abu Bakr officially appointed Muthanna commander in the Arabian Gulf area.

No sooner had Muthanna left Madina than he was joined by Khalid bin AI-Waleed at the head of an army 10,000 strong. When the two joined forces at the borders near the delta. Khalid sent a letter to Hormuz, the Persian governor and leader, offering him three options:
 

1. to embrace Islam; 
2. to pay tribute; 
3. to fight.

Hormuz was an intolerable man. He was disliked by the 'Iraqis who used to say of him: "There is no infidel more wicked than Hormuz ." Yet in Persia he was regarded as a nobleman of the highest rank. This was shown by his beret ¹⁴ which was evaluated  ¹⁵ at 100,000 dirhems.

 Getting no reply, Khalid divided his army into three regiments ¹⁶  of 6,000 men each. The first day Muthanna marched to Hafeer; the second day 'Adiyy bin Hatim followed; and the third day Khalid found Hormuz occupying the water springs.

Hormuz wanted to cut the fight short by murdering ¹⁷ Khalid treacherously. So, no sooner were the two armies engaged in battle than he challenged Khalid to fight a duel. Khalid at once dismounted ¹⁸  and before long his foe ¹⁹ was killed and Khalid cut off his head and held it by the forelock ²⁰ . The Persians dashed up at full gallop ²¹  to prevent their captain's death, but the Muslims were ready for them. The Persians took to their heels and a massacre followed. Among the spoils sent to Madina were Hormoz's beret and an elephant. The huge animal was publicly admired by old and young, and then returned to 'Iraq to be made use of in the forthcoming war. As for the warriors' booty, each gained about 1000 dirhems as well as the plundered ²²  arms.

The Battle of the Chains was so called because the Persian troops were tied together by chains. The heap ²³  of chains which was collected from the battlefield amounted to a camel load weighing 1000 pounds.

Muthanna pursued the retreating ²⁴  Persians for a long way, when suddenly he saw reinforcements coming to Hormuz from Mada'in, the capital. He sent news of the situation to Khalid and halted opposite the Persians at Mathar. Qarun, the Persian leader, wanted to fight Muthanna alone before the Muslims could come to his aid, but he was too late. As soon as Khalid was informed, he hastened to Mathar and engaged the enemy although he was ill-prepared . ²⁵  Qarun, Qabath and Anoshjan, the three enemy leaders, were all killed. Anything that the Muslim fighters could plunder in battle was kept by them and one fifth of the spoils was sent to Madina.

Being weakened by two successive defeats, Ardasheer, the Persian king, asked for the help of his Arab allies. A huge army was collected from the tribe of Bakr bin Wa'il, and in order to further restore the Persian forces, another army was also recruited ²⁶  The commander-in-chief this time was Bahman and the site of the battle was Walajah.

Khalid weighed up ²⁷  the situation and worked out his strategy ²⁸ . He left two battalions in ambush ²⁹ , so as to attack the enemy from the rear  ³⁰ at the right moment. The battle was furious and at the start it was doubtful if the Muslims would Win; but when the two battalions appeared, the fight was decided and the booty was unbelievable. Khalid's remark was: "Behold the incredible heap of spoils! We have attained two goals! Victory in God's cause, and amazing wealth."

This was the fiercest battle Khalid fought in 'Iraq. As the Banu Bakr wanted to avenge their defeat at Walajah, they invited all the Christians of' Iraq to oppose Khalid at Ollays. Also, Ardasheer gave orders to Bahman to lend them full support. However, because the king was ill, Bahman left his forces to visit him, leaving Jaban as leader in his place. Jaban was given strict orders not to engage the Muslims unless he was obliged to do so. The battle flared up and both sides fought patiently and bravely. When Banu Bakr's line began to waver Jaban was forced into battle.

 Expecting help from Bahman, the Persians proved stubborn  ³¹  and persistent. But because of the king's death, Bahman was unable to leave and no aid arrived at Ollays. The fight was so fierce that Khalid vowed if ever he won the battle, he would kill as many of his enemies as would make the nearby river flow ³²  with blood. So, when in the end, the Muslims got the upper hand, Khalid gave his men orders to take prisoners and send them to him. The river was diverted from its course, and some historians say that 70,000 non Muslims were put to the sword; but the blood did not flow. It was then suggested that the river revert to its normal course and this made the blood flow so Khalid's vow was fulfilled.

Maneeshya, a neighbouring town whose inhabitants took an active part in the fight, was pulled down and all the people's possessions were taken as booty. Each knight's share on that day amounted to 1500 dirhems. When Abu Bakr heard the details from Jandal, who was sent to Madina with the spoils, he said: All the women of the world are too barren  ³³  to give birth to a man like Khalid!"

 Al-Heerah had been the capital of the `Iraqi Arabs since the second century of the Christian era. After the battle of Ollays all the rebels took refuge there. Its Persian governor, Azathba, awaited Khalid's arrival anxiously outside the city, and his son diverted ³⁴  the course of the river in an attempt to prevent the Muslims from attacking the city by using Maneeshyan ships. Khalid actually tried to convey  ³⁵ his men by ship but failed because the water was too shallow. So, at the head of a cavalry detachment, he surprised the governor's son and reverted the water to its normal course. Seeing what had happened to his son, Azathba ran away leaving the city to its fate ³⁶ . Refusing to accept Islam or to pay tribute, the inhabitants resisted attack, but the fortresses were stormed ³⁷ . Five delegates met Khalid to negotiate peace terms. According to the terms of the agreement, they had to pay the Muslims 190,000 dirhems every year. However Khalid refused to sign the agreement unless Karamah, the sister of one of the delegates, was delivered to Shuwayl, an obscure Muslim warrior to whom the Prophet had promised the woman if AI-Heerah was ever taken by the Muslims. It was a very difficult condition because Karamah was then 80 years old. However, Karamah said: "Never mind! I will go to him! He is a fool who saw me when I was a beautiful girl. He thinks that beauty is permanent. Now I am old and he will accept a ransom instead."

When Shuwayl saw Karamah he agreed to take a ransom, but he refused to accept less than 1000 dirhems, which he was willingly ³⁸ given. Later he regretted his decision because his friends made fun of him for accepting such a small ransom. Then he asked Khalid for a larger sum, explaining that he had not known a number greater than a thousand. Khalid laughed and said: "That is OK ! You meant one thing but God meant something else!"

 After the peace treaty was concluded, similar treaties were signed between Khalid and other dignitaries ³⁹ ; and before long all the regions between the Arabian Gulf and Al-Heerah, which extended as far as the Tigris, passed under Muslim control.

 Al-Heerah was a convenient place for Khalid's headquarters. He remained there for a whole year without attempting to conquer Mada'in. In fact, he was following the instructions of Abu Bakr, who for bade him to leave Al-Heerah unless his comrade, `lyadh bin Ghanm, arrived after subduing Doomah, which was a long way to the south.

 But Khalid grew impatient and began to call it "a womanly year" as he was anxious to continue his conquests. He wanted to spread Islam everywhere. So he looked westwards along the banks of the Euphrates and saw Al-Anbar. Leaving Qa'qa' in Al-Heerah, he marched quickly to the city and lay siege to it. But, as it was surrounded by a ditch  ⁴⁰ , it could not be stormed. What could he do? He checked the ditch, and across the narrowest  ⁴¹ part of it he killed some lean camels and made a bridge of them. Soon the walls were scaled and the gates were broken open. To avoid another massacre, the Persian governor, Sheerzad, negotiated peace. He had to forfeit all his possessions but was allowed to leave at the head of a cohort ⁴²  of cavalry.

 It took Khalid three days to cover the distance between Al-Anbar and `Ayn AI-Tamr. There were Arabs and Persians waiting for him. The Arabs advised the Persians not to take part in the war as the Persians were not experienced at fighting Arabs - a piece of advice which the Persians thankfully accepted.

As soon as Khalid arrived, 'Oqqah, the Arab chief, challenged him to a duel - a challenge which Khalid readily accepted. It took Khalid only a few minutes to overpower ⁴³  `Oqqah, who was then taken captive. Seeing the consequences of the Arabs' obstinacy  ⁴⁴ , the Persians fled and the city gates were opened.

Doomat Al-Jandal is a strategically important place which lies about 300 miles to the south of 'Ayn Al -Tamr. It forms a juncture ⁴⁵  between Arabia,'Iraq and Syria. This is why Abu Bakr sent'Iyadh bin Ghanm at the head of an army to subjugate it. If it were not in the hands of the Muslims, their rear might be threatened at any time by the insurgents. `Iyadh spent a whole year trying unsuccessfully to subdue Doomat Al Jandal. Then he sent a message to Khalid asking for help. Khalid was overjoyed  ⁴⁶ to receive the message, and the next day he set off southwards. He covered the distance in ten days, only to find 'Iyadh besieging the fortified city and at the same time besieged by his enemies.

As soon as 'Okaydir, the governor of the city, heard of Khalid's arrival, he advised his allies to negotiate peace. His advice being rejected, he went to Khalid and disgracefully surrendered himself. He still remembered the lesson which Khalid had taught him during the life of the Prophet. There are two accounts of'Okaydir's end. One of them says that he was killed, and the other that he was sent to Abu Bakr, who kept him in jail until the caliphate of 'Omar set him free.

 As for the allies, a large number of them remained outside the city walls because there was no room for them inside. When Khalid attacked them, he killed Judi, their chieftain, first. Then he brought the captives to the city gate and killed them. They were so numerous that the gate was blocked by their bodies. Then he gave orders for the gate to be broken down. All the warriors inside were put to the sword, and the women were auctioned. He chose for himself the pretty daughter of the murdered Judi, with whom he stayed in Doomaat AI-Jandal for some time.

Though the Arab tribes were defeated many times, they did not stop plotting against Khalid. This forced him to march northwards to deal with their incessant plots and intrigues. The Banu Taghlib were dealt their severest blow when they were surrounded from three sides and their army completely annihilated.Having finished with the rebel tribes, Khalid marched on till he reached the border between Syria and 'Iraq. He camped there for a whole month while the Roman army was just opposite him across the border. Eventually the Romans felt they could no longer sit passively while the Muslims challenged them to their faces. They sent a message to Khalid inquiring whether he or they should cross the Euphrates. Khalid invited the Romans to cross, and then surprised them by his onslaught ⁴⁷  before they could finish crossing and take rest. Three well-known Muslim historians, Al-Tabari, Ibn AI-Atheer and Ibn Khaldun, agree that about 100,000 enemy soldiers fell in this battle.