Rasulu Allah (saws) sent a messenger to Banu Al Nadhir. The messenger informed them that Rasulu Allah (saws) had discovered their plot. He also informed them that they should leave Al Madinah within ten days as a punishment for them.
Banu Al Nadhir were very stubborn. They refused to evacuate Al Madinah. Instead, their reply was that they are not afraid of Muhammad and that they wanted to end the peace agreement they signed before. This left Rasulu Allah (saws) with no option but to fight them. So, he (saws) ordered the Muslim army to fight Banu Al Nadhir.