Jewish tribe of Banu Qurayza, whose forts were located in southern Medina, were caught conspiring to ally themselves with the confederates and were charged with treachery. After the retreat of the coalition, Muslims besieged their forts, and they were the last of the Jewish tribes of Medina. The Banu Qurayza surrendered and all the men and one woman were beheaded, apart from a few who converted to Islam, while all the other women and children were enslaved. In dealing with Muhammad's treatment of the Jews of Medina, aside from political explanations, western historians and biographers have explained it as "the punishment of the Medinan Jews, who were invited to convert and refused, perfectly exemplify the Quran's tales of what happened to those who rejected the prophets of old." Francis Edward Peters adds that Muhammad was possibly emboldened by his military successes and also wanted to push his advantage. Economical motivations according to Peters also existed since the poorness of the Meccan migrants was a source of concern for Muhammad. Peters argues that Muhammad's treatment of the Jews of Medina was "quite extraordinary" and is "quite at odds with Muhammad's treatment of the Jews he encountered outside Medina." According to Welch, Muhammad's treatment of the three major Jewish tribes brought Muhammad closer to his goal of organizing a community strictly on a religious basis. Siege of Khaybar In March 628, according to Muslims sources, the Jews of Khaybar, along with the Banu Nadir, who were exiled from Medina by Muhammad for violating the Constitution of Medina, and the Banu Ghatafan, were planning to attack the Muslims. When Muhammad learned of their alliance, he gathered an army of 1,500 soldiers and besieged the Jewish fortress at Khaybar. Scottish historian and orientalist, William Montgomery Watt agrees with this view. Italian orientalist Laura Veccia Vaglieri claims other motives pushed Muhammad to invade the forts of Khaybar. On the other side, the Banu Ghatafan were afraid that the Muslims would attack them at any time, so they refused to help the Jews at Khaybar. After capturing six of the eight Jewish forts in Medina, the Jews of Khaybar finally surrendered and were allowed to live in the oasis on the condition that they would give one-half of their produce to the Muslims. Two Jewish commanders were killed in the siege. They continued to live in the oasis for several more years until they were expelled