Voilation Of Treaty Done By Banu-Quraiza
They were bound according to the treaty of Madina, that they will put up a mutual defense against the foreign invading on Madina. But not only they did not support the Muslims during the invasion of Ahzab (The Battle of Ahzab) they betrayed the Muslims by threatening to attack them from inside Madina. you can find this in any book of seerah and early Muslim history.