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.

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