Did Muhammad break a truce to mercilessly conquer Mecca?
The Treaty of Hudaibiyah was a 10-year peace agreement—heavily in the favor of Meccans—signed in 628 between Prophet Muhammad and the Meccans. Prophet Muhammad represented the Muslims and the Medina State, Suhail bin Amr represented the polytheists and the Meccan state.
In addition to ensuring a decade of peace the Treaty held, “A young man, or one whose father is alive, if he goes to Muhammad without permission from his father or guardian, will be returned to his father or guardian. But if anyone goes to the Quraish, he will not be returned.”  Indeed, a lopsided provision.
Prophet Muhammad had not yet signed the treaty when Abu Jandal, son of Suhail bin Amr presented himself to Prophet Muhammad. Abu Jandal had been beaten and was in shackles for accepting Islam—he begged Prophet Muhammad to give him amnesty and protection. 
Suhail bin Amr protested, ‘Muhammad, the agreement between us was concluded before this man [Suhail bin Amr’s own son] came to you.’ Muhammad replied, ‘You are right.’ As Abu Jandal shrieked for help, Muhammad replied, ‘O Abu Jandal, be patient and control yourself, for God will provide relief and a means of escape for you and those of you who are helpless. We have made peace with them and we and they have invoked God in our agreement and we cannot deal falsely with them.’ 
Though Prophet Muhammad had not signed the agreement, he acknowledged that he had given his word to the terms written, and held fast to his word. And Prophet Muhammad was not merely attempting to gain some tactical advantage. When Abu Jandal soon after escaped Mecca and presented himself to Prophet Muhammad, once again seeking his protection, Prophet Muhammad sent him back to Mecca per the Treaty’s requirement.
Critics object that Prophet Muhammad granted amnesty to women who escaped persecution in Mecca and sought protection in Medina. Those critics who would dare compel women to live under a regime of persecution should re-evaluate their character. But even considering the black letter of the law, the Treaty explicitly stated that “any young man” must be returned. The subject “man,” not “person” is employed, and no clause accounted for women. Therefore, Prophet Muhammad was completely within legal rights to offer amnesty to women who sought his protection in Medina from persecution in Mecca. It should stand to reason, additionally, that if women fled Mecca and willingly sought protection under Prophet Muhammad, then it demonstrates just how much Islam elevated the rights and position of women as compared to their lives prior.
But back to the crux of this discussion—critics attempt to blame Prophet Muhammad for violating the Treaty of Hudaibiyah. The Treaty included a relatively common ally clause that extended the peace agreement to each party’s respective alliances. Thus, should a Muslim ally attack a Meccan ally, it would be considered a breach of the treaty—and vice versa. Not two years after the Treaty was signed, a Meccan ally, the Banu Bakr, attacked a Muslim ally, the Khuza‘a, and killed several of their men. 
Upon learning that the Khuza‘a sent a delegation to Prophet Muhammad to report the breach, the Meccans sent Abu Sufyan as an emissary to convince Prophet Muhammad to renew the Treaty.  Prophet Muhammad declined to renew the treaty for obvious reasons—to agree to an unfair treaty is one thing, but to oblige oneself to another unfair treaty in the face of a breach that cost human life is beyond injustice.
On the day Prophet Muhammad marched back to Mecca with 10,000 soldiers, the city in the palm of his hand, the opportunity to exact vengeance on any person for any reason for all the wrongs they had done to him, at that moment this alleged unjust warmonger offers, peace?
Washington Irving, by no means an Islamic apologist, is forced to admit:
The sun was just rising as he entered the gates of his native city, with the glory of a conqueror, but the garb and humility of a pilgrim. He entered, repeating verses of the Koran, which he said had been revealed to him at Medina, and were prophetic of the event. He triumphed in the spirit of a religious zealot, not a warrior. 
Stanley Lane Poole likewise adds:
The day of Muhammad’s greatest triumph over his enemies was also the day of his grandest victory over himself. He freely forgave the Quraish all the years of sorrow and cruel scorn to which they had inflicted him, and gave an amnesty to the whole population of Mecca. 
Find any comparable example where a man is beaten repeatedly, exiled from his native land, pursued to be murdered, assassination attempts are made on him on no less than five occasions, his family members mutilated and their corpses literally eaten, his companions dragged in the streets, the unfair peace treaty he signs savagely broken—and in response, on the occasion of his victory after two decades of persecution, as the undisputed king of the land—he offers amnesty to all those who persecuted him. He demands no reparations, no apologies, no enslavement—but instead offers forgiveness. No such example exists other than that of Prophet Muhammad’s and we challenge any critic of Islam to prove otherwise.
In fact, Prophet Muhammad specifically asked the Meccans:
What punishment should you have for the cruelties you committed against those whose only fault was that they invited you to the worship of one God?” The Meccans replied, “We expect you to treat us as Joseph treated his erring brothers.” Prophet Muhammad smiled and replied, “No blame shall lie upon you today. You are free to go.” 
As the de facto ruler of Arabia, Prophet Muhammad and his allegedly “merciless” army forgave all their tormentors and set them free.
Having established that Prophet Muhammad did not break the Treaty, and that he demonstrated unmatched compassion upon returning to Mecca, we are compelled to add Abu Sufyan’s testimony regarding Prophet Muhammad. After all, Abu Sufyan was the chief of the largest Meccan tribe and leader of their forces. Thus, any fair-minded reader would agree that Abu Sufyan’s testimony—as leader of the allegedly persecuted Meccans and ostensibly the leading opposition to Prophet Muhammad—carries significant weight.
Throughout his prophethood, Prophet Muhammad sent letters to numerous kings and emperors of his time, inviting them to accept Islam. Perchance, Abu Sufyan was in Ghaza, near Jerusalem, where Heraclius Caesar received Prophet Muhammad’s letter. This incident occurred after the Treaty of Hudaibiyah was signed, but before the Meccans violated it. Caesar held a grand court and invited Abu Sufyan to approach. When Abu Sufyan approached, Caesar questioned him about the Prophet: 
Caesar: To what sort of family does the Claimant to prophethood belong?
Abu Sufyan: A noble one.
Caesar: Did someone else in his family claim Prophethood?
Abu Sufyan: No.
Caesar: Did someone from his family happen to be a king?
Abu Sufyan: No.
Caesar: Are the people who have accepted this religion poor or influential?
Abu Sufyan: They are poor people.
Caesar: Are his followers growing in number or decreasing?
Abu Sufyan: They are growing.
Caesar: Have you ever experienced untruthfulness from him?
Abu Sufyan: No.
Caesar: Does he ever break a promise or agreement?
Abu Sufyan: Thus far he has never done it. But it is seen if he adheres to the new peace pact which has just been concluded.
Caesar: Have you ever fought a battle against him?
Abu Sufyan: Yes.
Caesar: What was the result of the battle?
Abu Sufyan: Sometimes we were victorious and sometime he was successful.
Caesar: What does he teach?
Caesar: Did anyone else make such a claim before him in his people?
Abu Sufyan: No.
Later Abu Sufyan exclaimed, “By God! Throughout the conversation, except the statement [regarding the new pact], I did not get any chance to say anything against Muhammad.” The discussion continued and Heraclius Caesar then explained why he asked those particular questions.
Caesar: When I asked you about the lineage of the claimant of the prophethood, you stated that he belongs to a very noble family. Messengers always belong to noble families. I asked you if there has been a king in his forefathers? You responded, No. From this I concluded that had there been a king in his forefathers, he might be desirous of regaining the kingdom of his forefathers. I asked you about his followers, whether they are rich and powerful? You replied they are weak and poor. In the beginning, always the poor and weak accept the messengers. I asked you, Did you ever blame him for telling a lie before he claimed to be a prophet? You said, “No.” I was convinced that the one who does not tell a lie to the people, how can he tell a lie about God? Then I asked you, Did anyone of his followers apostatize after accepting Islam due to disliking Islam? You said, “No.” This is the case with a true faith. When someone accepts a faith with clarity of mind, it is very difficult for him to turn away from that faith. I asked you, whether they are increasing or decreasing in number? You said that they are increasing in number and also in steadfastness. This is always the case with true faiths. I asked you, Did you ever fight a battle with him? You said, We have fought several battles. Sometimes they had the upper hand in the battle and other times we had the upper hand. Sometimes we were successful while the other times they were successful. This is the case with the messengers of God. In the beginning, they went through many trials but ultimately they were triumphant. I asked you, Did he ever break an agreement or deal treacherously? You said, “No.” Such is the high status of the prophets. They never break an agreement. Then I asked, has anyone among your people claimed to be a prophet before him? You said, “No.” From this, I concluded that since there had not been a prophet in his people he is not imitating anyone.
Abu Sufyan said then, Heraclius asked him, “What did [Muhammad] command you to do?” I said, “He commanded us to observe Prayer, pay Zakat, strengthen the ties of kinship, tell the truth, be pious and chaste.” Hearing this Heraclius said,
If everything you have told is true, then definitely he is a prophet. I was expecting the coming of a prophet. However, I did not know that the prophet would be commissioned from among your people. Had the circumstances permitted me, I certainly would have gone to see this prophet. Had I visited him, I would have washed his feet. The kingdom of this prophet will reach the land where I stand.
Heraclius Caesar then asked for Prophet Muhammad’s letter and read it before his court.
Abu Sufyan, the leader and general of the Meccans, cannot help but admit that Prophet Muhammad was truthful, just, honest, and noble. Heraclius Caesar cannot help but admit Prophet Muhammad was indeed God’s true prophet, dignified and honorable. This is the true story of Prophet Muhammad’s life and victory at Mecca. In short, Prophet Muhammad broke no treaty, and let alone with justice, he acted with unprecedented compassion upon returning to Mecca. This is what even his enemies readily admitted to then, and this is what non-Muslim scholars admit to today.
 Id. at 88.
 Muslim Chap. 33, Book 19, #4404.
 Id. at 505.
 Id. at 234.
 Washington Irving, Mahomet and His Successors, Vol. 1,253 (New York: G.P. Putman’s Sons, 1868).
 Ibn Hisham, Tuwaf al-Rusul Bi al-Bait wa Dukhulih Al-Haram. Also Ibn Kathir, Ibn Sa‘d, Zurqani, Hilbiya under Victory of Mecca.
 Bukhari, Vol. 4, Book 52, #191.