The Treaty of Hudaybiyya had included a vow of there being no fighting between the Quraysh and the Muslims. However, when the tribe of Banu Bakr asked the leaders of Quraysh for help in a raid against their long time foe Banu Khuza'ah, a tribe with many Muslims and who were longtime allies of Prophet Muhammad, the Quraysh joined in.
The raiders killed 23 members of Banu Khuza'ah. The tribe went to Prophet Muhammad for help and the Prophet pledged to attack Mecca in resopnse.
Meanwhile Abu Sufyan learned about the raid and panicked realzing that it meant the Quraysh had violated the treaty. He went to Medina, to try to apologize. The same Abu Sufyan who used to be relentless in persecuting the Muslims was now begging for protection.
But the deaths and the violated treaty could not be taken back.
Lecture notes available at http://www.why-quran.org/?p=673.
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