In the year 6 Hijri, a treaty was concluded (treaty of Hudaybiyah) with the leaders of Quraysh and Prophet Muhammed. According to the pact, Quraysh and the Muslims were free to make agreements with the tribes they liked.
As a result, the Banu khuza’a tribe decided to make a pact with the Muslims. And the Banu Kinana (Banu Bakr), who were old enemies of the Khuza’a sided and made agreement with the Quraysh. Part of the agreement in these treaties was that there will be peace throughout Arabia i.e., that the parties who agreed to the pact will not fight fight against each other, nor incite or instigate any war.
While there was general peace for few years, the Muslims busied themselves with worship and preaching the message of Islam. The old feud started again between the Khuza’a and Banu Bakr b. Abdu Manat b. Kinana. The Quraysh instigated the banu Bakr in fighting against Khuza’a, as a result many men of the Khuza’ah were killed.
As a consequence of the Quraysh’s involvement and the killing of the Banu Khuza’ah’s people, the conquest of Makkah took place.
“THE CAUSES THAT LED TO THE OCCUPATION OF MECCA, A.H. 8
After he had sent his force to Mu’ta the apostle stayed in Medina during the latter Jumada and Rajab. Then the B. Bakr b. Abdu Manat b. Kinana attacked Khuza’a while they were at a well of theirs in the lower region of Mecaa called al-Watir. The cause of the quarrel was that a man of B. al-Hadrami called Malik b. Abbad – the Hadramu being at that time allies of al-Aswad b. Razn – had gone out on a trading journey; and when he reached the middle of the Khuza’a country they attacked and killed him and took his possessions.
So B. Bakr attacked a man of Khuza’a and killed him; and just before Islam Khuza’a attacked the sons of al-Aswad b. Razn al-Dili who were the most prominent chiefs of b. Kinana – Salma, Kulthum, and Dhu’ayb – and killed them in Arafa at the boundary stones of the sacred area.
One of the B. al-Dil told me that B. al-Aswad during the pagan era were paid double bloodwit because of their position among them, while they only got a single bloodwit. While B. Bakr and Khuza’a were thus at enmity Islam intervened and occupied men’s minds. When the peace of Hudaybiya was concluded between the Apostle and Quraysh one of the conditions – according to what al-Zuhri told me from Urwa b. al-Zubayr from al-Miswar b. Makhrama and Marwan b. al-Hakam and others traditions – was that anyone who wanted to enter into a treaty relationship with either party could do so; the B. Bakr joined Quraysh and Khuza’a joined the Apostle. When the armistice was established B. al-Dil of B. Bakr took advantage of it against Khuza;a in their desire to revenge themselves on them for the sons of Aswad whom they had killed. So Naufal B. Mu’awiya al-Dili, who was their leader at the time, went out with the B. al-Dil, though all the B. Bakr did not follow him, and attacked Khuza’a by night while they were at al-Watir their well, killing one of their men. Both parties fell back and continued the fight. QURAYSH HELPED B. BAKR WITH WEAPONS AND SOME OF THEM FOUGHT WITH THEM sexretly under cover of the night until they drove Khuza’a into sacred area. When they reached it the B. Bakr said, ‘O Naufal, we are in the sacred area. Remember your God, remember your God!’ He replied in blasphemous words that he had no god that day. ‘Take your revenge, ye sons of Bakr. By my life, if you used to steal in the sacred area, wont you take vengeance in it?’
Now on the night they attacked them in al-Watir they killed a man called Munabbih who had gone out with one his tribesmen called Tamim b. Asad. Munabbih had a weak heart and he told Tamim to escape for he was as good as dead whether they killed him or let him go, for his heart had given out. So Tamim made off and escaped and Munabbih was overtaken and killed. When Khuza’a entered Mecca they took refue in the house of Budayl b. Warqa and the house of a freed slave of their called Rafi. … When Quraysh and B. Bakr had combined against Khuza’a and killed some of the, thereby breaking their covenanted word with the Apostle in violating Khuza’a who were in treaty with him, Amr b. Salim al-Khuza’a of the clan of B. Ka’b went to the Apostle in Medina. (This led to the conquest of Mecca).”(Ibn Ishaq, page 540 – 542) 
Another report, Zad al-Ma’ad:
“The Quraysh helped Banu Bakr with weapons while their chiefs, taking advantage of the night, fought Khuz’a along with Banu Bakr. Their combined charge drove Khuza’a into the sacred territory where some of the Qurayshites said to one another: ‘We are now in the sacred area. Mind your gods! Mind your gods!’ But others replied imprudently: ‘We have no god today. Take your revenge, O sons of Bakr, for you may not get a chance again.” (Zad al-Ma’ad, volume 1, page 419, and Ibn Hisham, volume 2, page 390) 
Shaykh Muhammad al-Ghazali:
“THE CONQUEST OF MAKKAH
After the Treaty of Hudaybiyah the Muslims busied themselves with the spread of the Da’wah and the presentation of the teachings of Islam to all who had understanding. Their faithfulness to the Quraish was visible to all in matters that they liked or disliked and the people could see clear evidence of that. However, the Quraish persisted in their old, stagnated policy without taking into account the momentous events which had changed conditions in the Arabian peninsula and which were about to change conditions in the world as a whole. This lack of insight led the Quraish to commit a grave act of stupidity, after which the treaty of Hudabiyah was to become null and void. It happened that the Quraish, along with their allies of the Banu Bakr, attacked the Khuza’ah, who were allies of the Muslims, and killed a few of their men.
The Khuza’ah sought refuge in the sanctuary of the Ka’bah for they were not prepared for combat. Despite that, the Banu Bakr pursued them while the Quraish lent arms and assisted the Banu Bakr in their oppression.
Some men of the Banu Bakr felt that they ought not to fight in the Haram, and spoke to their chief, Naufal ibn Mu’awiyah, about this, saying: ‘We have entered the Haram, (so mind) your god, your God!’
Nevertheless, Naufal said: ‘There is no God today, Banu Bakr, Carry out your task!’
The Khuza’ah were terrified by what happened to them, and they sent Amir ibn Salim to the Prophet to tell him the news. Upon hearing the account, the Prophet promised to come to their aid. … The Prophet ordered the people to prepare themselves and said that he was going to Makkah. He urged them to be serious and brisk. He supplicated: ‘O Allah, take away the eyes and the news from the Quraish until we surprise them in their land.’ The Muslims listened to the Prophet’s order and began mobilizing their forces for the expected encounter. They knew that the decisive hour between them and the Quraish was close. 
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 Ibn Ishaq’s Sirat Rasul Allah – The Life of Muhammad Translated by A. Guillaume, page 540 – 542
 Muhammad Rasulullah: The Apostle Of Mercy, [Translated by Mohiudin Ahmad, Academy of Islamic Research And Publications, Lucknow (India) – Series No. 126 – Edition English 2nd Lucknow, 1982]S. Abul Hasan Ali, page 314
 Fiqh-us-seerah, Shaykh Muhammed al-Ghazali, page 406