Prophet Muhammad approached the Jewish tribe of Banu Nadhir to help the Muslims make peace between themselves and another Jewish tribe, only to discover at the last minute that Banu Nadhir was planning to assassinate him.
Why? The Banu Nadir couldn't accept a gentile as the final messenger of God. They considered it degrading to be subservient to a non-Jew.
Faced with this treachery, Prophet Muhammad commanded them to take their belongings and leave Medina in ten days.
The Banu Nadhir initially refuse, preparing for war, but surrender once the Muslims besieged their fortress. Despite their willingness to fight the Prophet still allowed the tribe to depart, but orders them to leave their weapons behind this time. Banu Nadhir reloate to the Fortress of Khaybar
Surah Al Hashr (#59) verses 1-16 describe this encounter.
Lecture notes available at http://www.why-quran.org/?p=524.
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