After the battle of Uhud, Abu Sufyan and the other pagan leaders realized that they had fought an indecisive action, and that their victory had not borne any fruits for them. Islam had, in fact, resiled from its reverse at Uhud, and within an astonishingly short time, had reestablished its authority in Medina and the surrounding areas.

The pagans considered Islam a threat to their economic security and political supremacy in Arabia, and they could never be reconciled to its existence. They knew that if they could kill Muhammad, their interests would be safeguarded, and their hegemony would be restored in Arabia. With this aim they decided to strike a final and a crushing blow upon Medina, and to exterminate all Muslims.

Montgomery 

The strategic aim of the Meccans was nothing less than the destruction of the Muslim community as such, or – what amounts to the same thing – the removal of Muhammad from his position of authority (Muhammad, Prophet and Statesman)

Inspired by this aim, and by their ardor to make restitution for failures of the past, the Makkan leaders began preparations for an all-out war; a war that would put an end to all other wars by blotting Islam out!

In two years the Quraysh raised a fighting force of ten thousand warriors. This was the largest force ever assembled by the Arabs till that time. With great fanfare and aplomb, this formidable force left Makkah in February 627 to capture Medina and to obliterate Islam.

Muhammad Husayn Haykal

When news of this tremendous mobilization reached Muhammad and the Muslims in Medinah, it struck them all with panic. The mobilization of the whole of Arabia against them instilled fear in their hearts as they faced the prospect of being not only defeated but wiped out. The gravity of the situation was evident in the fact that the army the Arab tribes had now raised surpassed in number and equipment anything the Peninsula had ever seen before... (The Life of Muhammad, Cairo, 1935)

The Prophet convened an emergency meeting of his principal companions to consult them in the matter of defending the city. One thing was obvious. The Muslims were so few in number and so poor in equipment that they could not meet the invading force on the open ground. Medina had to be defended from within. But how? How could the tiny Muslim garrison prevent the Makkan army from overrunning Medina which would be overwhelmed by sheer numbers, was a question on everyone's mind.

One of the closest friends of Muhammad, the Messenger of God, was Salman the Persian. He was born and brought up in Persia (Iran) but had spent many years in Syria and Palestine, and he had familiarity with the warfare and the siege operations of both the Persians and the Romans. Medina had natural or man-made defenses on three sides but was exposed on one, i.e., the north side. Salman told the Prophet that if a trench were dug on the north side, the city could perhaps be defended successfully.

The idea, though new and unconventional in Arabia, appealed to the Prophet. He accepted it and ordered Muslims to dig the trench.

Muhammad Husayn Haykal

Salman al-Farsi, who knew far more of the techniques of warfare than was common in the Peninsula, advised the digging of a dry moat around Medina and the fortifications of its buildings within. The Muslims hurried to implement this counsel. The moat was dug and the Prophet – may God's peace and blessings be upon him – worked with his hands alongside his companions lifting the dirt, encouraging the Muslim workers, and exhorting everyone to multiply his effort.  (The Life of Muhammad, Cairo, 1935) 

Since the Makkan army was known to be approaching Medina rapidly, there was no time to lose, and the Muslims worked frantically – in relays. In six days the trench was dug, just in time to prevent the invaders from taking the town by assault. 

The Makkan cavalry came like a whirlwind but was suddenly checked, in its career, by the trench. The horsemen reined in their horses at its edge. Their grand strategy had been to take Medina by storm in a few hours but now it appeared to them that they could not do so. Here there was a trench, a new obstacle which they could not surmount. How did it fit into their strategy? They were utterly nonplused by the trench.

Eventually, and after long deliberation, the Makkan commanders decided to lay siege to Medina, and to force the Muslims to surrender, through attrition. They sealed all exits from Medina, and hemmed in the Muslims. Medina was in a state of siege!

Though it was Abu Sufyan who had organized the whole campaign, and he was its director of operations, he was no fighting man himself. The fighting man of his army was Amr ibn Abd Wudd, the fiercest of the warriors of pagan Arabia. Abu Sufyan's hopes of a swift and decisive victory over the Muslims lay in him. M. Shibli, the Indian historian, and Abbas Mahmood Al-Akkad, the Egyptian historian, say that Amr ibn Abd Wudd was reckoned, by the Arabs of the time, to be more than a match for one thousand cavaliers.

Amr ibn Abd Wudd had no interest in the static warfare of a siege. He panted for action. When a few days had passed, and nothing had happened, he lost patience, and he decided to capture Medina by personal action. One day, prowling around Medina, he and three other Makkan knights discovered a rocky point where the trench was not too wide. They spurred their horses from it, and succeeded in clearing the trench!

Now Amr was inside the perimeter of Medina. He boldly advanced into the Muslim camp, and challenged the heroes of Islam to come out and fight against him in the classical Arabian tradition of duels.

Amr's first challenge went unanswered whereupon he repeated it but still got no answer. Such was the prestige of his name that no one in the Muslim camp dared to meet him in a trial of strength. If the idolaters saw in him their hope of victory, the Muslims saw in his challenge the sentence of their death.

Amr ibn Abd Wudd threw his insolent challenge a third time and taunted the Muslims at the same time for their cowardice.

To Amr it must have seemed that the Muslims were paralyzed with fear, which most of them, in fact, were. Al-Qur’an al-Majid has also drawn a portrait of the state of the Muslims at the siege of Medina in the following verses:

Behold! They came on you from above you and from below you, and behold, the eyes became dim and the hearts gaped to the throats, and you imagined various (vain) thoughts about God! (Chapter 33; verse 10)

Behold! A party among them said: "you men of yathrib! You cannot stand (the attack). Therefore go back" and a band of them asked for leave of the Prophet saying, "truly our houses are bare and exposed." Though they were not exposed: they intended nothing but to run away. (Chapter 33; verse 13)

Amr ibn Abd Wudd even expressed amazement that the Muslims were not showing any eagerness to enter paradise where he was ready to send them.

It is true that most of the Muslims were terror-stricken but there was one among them who was not. He had, in fact, volunteered to accept Amr's very first challenge but the Prophet had restrained him, hoping that someone else might like to face him (Amr). But he could see that no one dared to measure swords with him.

The young man who was willing to take up Amr's challenge was no one other than Ali ibn Abi Talib, the hero of Islam. When Amr hurled his third challenge, and no one answered him, Ali rose and solicited the Prophet's permission to go out and to fight against him.

The Prophet of Islam had no choice now but to allow his cousin, Ali, the Lion of Islam, to go and to silence the taunts and the jibes of Amr ibn Abd Wudd.

Ali put on the battle-dress of the Prophet of Islam. The latter himself suspended the Dhu'l-Fiqar to his side, and prayed for his victory, saying: "O Allah! Thou hast called to Thy service, Obaida ibn al-Harith, on the day the battle of Badr was fought, and Hamza ibn Abdul-Muttalib, on the day the battle of Uhud was fought. Now Ali alone is left with me. Be Thou his Protector, give him victory, and bring him back safely to me."

When the Prophet saw Ali going toward his adversary, he said: "He is the embodiment of all Faith who is going to an encounter with the embodiment of all Unbelief."

A few moments later, Ali was standing before Amr. The two heroes identified themselves, and sized up each other. Ali had a set of principles which he applied in all situations whether of war or of peace. In the battle of the Trench, the Muslims and the pagans saw a demonstration of the application of those principles. Whenever he confronted an enemy, he offered him three options. They were:

1.Ali presented Islam to his opponent. He invited him to abandon idolatry and to accept Islam. This invitation made Ali a missionary of Islam in the battlefield itself.

2.If the enemy did not accept Ali's invitation to accept Islam, he advised him to withdraw from the battle, and not to fight against God and His Messenger. Fighting against them, he warned him, would only bring eternal damnation upon him in the two worlds.

3.If the enemy did not accept the second option also, and refused to withdraw from the battle, then Ali invited him to strike the first blow. Ali himself was never the first to strike at an enemy.

Amr ibn Abd Wudd disdained even to consider the first and the second options but accepted the third, and struck a mighty blow with his ponderous sword which cut through the shield, the helmet and the turban of Ali, and made a deep gash in his forehead. Blood leapt out from the wound in a jet but Ali was not dismayed. He rallied, and then struck a counter-blow with the famous Dhu'l-Fiqar, and it cleft the most formidable warrior of Arabia into two!

When Amr was killed, the three knights in his entourage turned round and spurred their horses to retreat. Ali let them retreat. It was one of his principles not to pursue a fleeing enemy. Whoever wished to save his life, Ali let him save it.

The death of Amr ibn Abd Wudd broke the back of the Makkan offensive against Islam, and destroyed their morale. The elements also declared against them. The temperature fell to freezing point, and a dust storm arose which blew in their faces. Discouraged and disheartened, the fickle tribesmen began to desert their Makkan allies, first in ones and twos and threes, and then in tens and twenties, and a little later, in hundreds. The confederacy began to dissolve visibly. Abu Sufyan was compelled to raise the siege, and to give the signal to his army to retreat from Medina. His army was dispersed, and his campaign was a dismal failure. Medina was saved.

The failure of the siege of Medina by the idolaters of Makkah was a most significant event in the history of Arabia. It meant that they would never be able to mount another invasion of Medina. After the battle of the Trench, the initiative passed, finally and unmistakably, from the polytheists of Makkah to the Muslims of Medina.

Medina and Islam had been saved by an idea and a hero. The idea was the trench which immobilized the Makkan cavalry. It was an entirely new concept in Arabian warfare, and the Arabs had no familiarity with it. Without the trench, the ten thousand marauding tribesmen would have overrun Medina, and they would have killed everyone in it. The honors for saving Medina-tun­Nabi, the City of the Prophet, and the Capital of Islam, go to Salman the Persian, and to his master, the Prophet himself. The former broached a new idea in military doctrine; the latter showed himself receptive to it, and immediately implemented it. 

Everyone in Medina claimed that he was a friend or companion of Muhammad, the Messenger of God. That city had its own share of tuft-hunters. But there were a few, in fact very few, men whom Muhammad himself acknowledged as his friends. Salman the Persian belonged to this select group, the inner circle of the friends of the Messenger of God.

Salman was a man of gigantic stature and prodigious physical strength. When the trench was being dug, he worked as much as six other men. This prompted one of the Muhajireen to claim that Salman was one of them, i.e., the Muhajireen. But he was at once challenged by the Ansar one of whom said that Salman was an Ansar and not a Muhajir. The two groups were still arguing when the Apostle arrived on the scene. He too heard the claims of both sides and was amused by them. But he put an end to the argument by giving his own "verdict". He said that Salman was neither a Muhajir nor an Ansar but was a member of his own house – his Ahlul-Bait – a member of the House of Mohammed Mustafa himself!

The Arab historian, Ibn Atheer, has quoted the Prophet in his book, Tarikh Kamil, vol. 2, p. 122, as saying: "Salman is one of us. He is a member of our household." This is the greatest honor ever bestowed upon any of his companions by Muhammad, the Messenger of God.

Salman was a Christian living in Ammuria in Asia Minor when he first heard vague reports of the appearance of a prophet in Hijaz. To verify these reports, he came to Medina. When his first glance fell on the face of the Prophet, he exclaimed: "This cannot be the face of a man who has ever told a lie," and he forthwith accepted Islam.

Islam adopted Salman as much as he "adopted" Islam. Islam became the synthesis of his emotions, and he became a part of its "blood-stream." In Medina, a stranger once asked him the name of his father. His answer was: "Islam! The name of my father is Islam. I am Salman the son of Islam." Salman "blended" into Islam so thoroughly that he became indistinguishable from it. 

The threat to the security of Medina, however, did not pass with the digging of the trench. Medina was still vulnerable. At a point where the trench was narrow, the general of the Makkan army and three other champions, were able to leap over it and to ride into the Muslim camp. If they had succeeded in establishing a bridgehead over the trench, the whole Makkan cavalry and infantry, and the irregular freebooters would have entered the city and captured it. But Ali checkmated them. Thus the wits of Salman, the sagacity of Muhammad and the sword of Ali proved to be the best defense of Islam against the most formidable coalition of the polytheists in the history of Arabia. 

It was a custom in Arabian warfare to rob a vanquished foe of his weapons, his armor and his horse. At the siege of Medina, Amr was wearing the finest armor in all Arabia. Ali killed him but did not touch anything that belonged to him to the great surprise of Umar bin al-Khattab. Later, when Amr's sister came to his corpse to mourn his death, she too was surprised to notice his weapons and armor intact. When she was told that it was Ali who had killed him, she composed some verses praising him (Ali). These verses have been quoted by the Egyptian historian, Abbas Mahmood Al-Akkad, in his book, Al-Abqariyyat Imam Ali (the Genius of Imam Ali), and can be roughly translated as follows:

"If someone other than Ali had killed Amr,

I would have mourned his death all my life. 

But the man who killed him is a hero and he is peerless. 

His father was also a lord."

Commenting upon these lines, Abbas Mahmood Al-Akkad says that a tribe did not consider it a disgrace if any of its heroes was killed by Ali. Ali was the most gallant and most chivalrous of foes, and also he was invincible.

After the failure of the siege of Medina, all the tribes between Medina and the Red Sea and between Medina and Yammama to the east, signed treaties of peace with the Prophet of Islam.

In the same year, i.e. in 5 A.H. (A.D. 627), Hajj (pilgrimage to Makkah) was made mandatory for all those Muslims who were in good financial standing and were in good physical health.