You have not established that the Bani Qurayza broke a treaty with Muhammad.

In 627, a Meccan army under the command of Abu Sufyan, together with contingents from the Bedouin tribe of Ghatafan and the exiled Banu Nadir, marched against Medina and laid siege to it. According to Al-Waqidi, the Banu Qurayza helped the defense effort by supplying spades, picks, and baskets for the excavation of the defensive trench.[15] They also possessed large numbers of weaponry, as 1,500 swords, 2,000 lances, 300 suits of armor, and 500 shields were later seized by the Muslims.[19] Although the Qurayza did not commit any act overtly hostile to Muhammad,[3] there are reports about their negotiations with the Meccans:

Ibn Ishaq writes that during the siege Huyayy ibn Akhtab, the chief of the exiled Banu Nadir, came to the Qurayza chief Ka'b ibn Asad and persuaded him to help the Meccans conquer Medina. Ka'b was, according to Al-Waqidi's account, initially reluctant to break the contract and argued that Muhammad never broke any contract with them or exposed them to any shame, but decided to support the Meccans after Huyayy had promised to join the Qurayza in Medina if the besieging army would return to Mecca without having killed Muhammad.[20] Ibn Kathir and al-Waqidi report that Huyayy tore into pieces the agreement between Ka'b and Muhammad.[3][21]

Watt writes that Muhammad "became anxious about their conduct and sent some of the leading Muslims to talk to them" and that he considered "the result was disquieting."[3] According to Ibn Ishaq, Muhammad ordered a man from the Ghatafan who had secretly converted to Islam to go to Muhammad's enemies and sow discord among them. The man went to the Banu Qurayza and advised them to join the hostilities against Muhammad only if the besiegers provide hostages from among their chiefs. When the representatives of the Quraysh and the Ghatafan came to the Qurayza, asking for support in the planned decisive battle with Muhammad, the Qurayza indeed demanded hostages, breaking down the negotiations.[22] As a result, the Banu Qurayza did not take any action in support of the besieging army until Abu Sufyan's forces retreated.[15]
http://en.wikipedia.org/wiki/Banu_Quray ... the_Trench