Would you not fight people who violated their treaties, tried to banish the messenger, and they are the ones who started the war in the first place? Are you afraid of them? God is the One you are supposed to fear, if you are believers.
أَلا تُقٰتِلونَ قَومًا نَكَثوا أَيمٰنَهُم وَهَمّوا بِإِخراجِ الرَّسولِ وَهُم بَدَءوكُم أَوَّلَ مَرَّةٍ أَتَخشَونَهُم فَاللَّهُ أَحَقُّ أَن تَخشَوهُ إِن كُنتُم مُؤمِنينَ
alaa tuqaatiloona qawman nakasooo aimaanahum wa hammoo bi ikhraajir Rasooli wa hum bada’ookum awwala marrah; atakhshawnahum; fallaahu ahaqqu an takhshawhu in kuntum mu’mineen