They were under an oath to support the Muslims if an outside...

They were under an oath to support the Muslims if an outside force attacked al-Madinah. Abū-Sufyan was worried that if Banū-Qurayzah sided with the Muslims in terms of their agreement, and then their strength would increase. He wanted to try to make them repudiate the agreement. Therefore, he deputed one chief of Banū-al-Nadir, Huyay ibn Akhtab to visit Banū-Qurayzah and persuade them to violate the agreement.

Huyay went to the fortress of the chief of Banū-Qurayzah, Ka`b ibn Asad that was situated on the East of the city of al-Madinah. He knocked at the entrance and Ka`b asked who it was? When Huyay identified himself, Ka`b understood his sly purpose. He refused to open the door and admit him. Huyay insisted on him to open the door and told him that he was bringing for him a happy tiding that the Quraysh and all the tribes of Arabia had united to fight with the Muslims!

He added that if he wanted the goodwill of the Arabs, he should support their cause. Ka`b said that they had never experienced anything from the Prophet (a.s) other than good and trustworthiness! He said they would not breach the agreement needlessly. He also added that the unity of the Arabs he talked of was like the fast flying clouds that might thunder, but seldom rain! Huyay said that keeping the door closed for a guest is not a way with Arabs! Please open the door and talk to me for a while!

On the insistence of Huyay, Ka`b opened the door and admitted him. They entered into an argument and the result was that Huyay was able to talk Ka`b into agreeing to cooperate with them. He also assured him that in the event of the Quraysh losing, his tribe would face the situation with Banū-Qurayzah. Therefore, they tore away the document of agreement the tribe had signed with the Prophet (a.s). Now, Banū-Qurayzah were friends and allies of the Quraysh.

When the Prophet (a.s) learnt about the breach of agreement by Banū-Qurayzah, he sent Sa`d ibn Mu`adh to them to persuade them to behave. But this had no effect on them and they said in clear terms that they did not know any one nor did they have any agreement with anybody. Since these people were living within the boundaries of al-Madinah, they would be a danger for the women and children when hostilities with the enemy troops started. Their attitude added pressure on the Muslims.