Multiple factors came together to demoralize the Qurayshi coalition and got them to retreat, ending the battle:
- Imam Ali's victory over Amr ibn al-Wadd - Shortage of food and fodder. They had prepared for quick battle, not a month-long seige - A violent storm that trashed the Qurayshi camp - Distrust sown between the kuffar and the jews by a secret convert - Some Arabs deciding they'd rather be ruled by Arab than by Jews, even of those Arabs are Muslims
Lecture notes available at http://www.why-quran.org/?p=586.
Subscribe at http://www.why-quran.org/subscribe to watch the lectures live and participate in the Q&A at the end of each class.
Video recording of this lecture + Q&A available on YouTube at https://www.youtube.com/watch?v=IL9k-WuTfFE&list=PLpkB0iwLgfTat-Pgh4W3WFmupPamiC9UT.