Translation:

12. But if they violate their oaths after their covenant and taunt you for your faith fight ye the chiefs of unfaith: for their oaths are nothing to them: that thus they may be restrained.

Notes (Tafseer)

1260. Not only did the enemies break their oaths shamelessly, but they even taunted the Muslims on their Faith and the "simple-minded" way in which they continued to respect their part of the treaty, as if they were afraid to fight!