To let them live — That is, they should not destroy them. That this league was lawful and obliging, appears, Because Joshua and all the princes, upon the review concluded it so to be, and spared them accordingly. Because God punished the violation of it long after, 2 Samuel 21:1. Because God is said to have hardened the hearts of all other cities, not to seek peace with Israel, that so he might utterly destroy them, Joshua 11:19-20, which seems to imply that their utter destruction did not necessarily come upon them by virtue of any peremptory command of God, but by their own obstinate hardness, whereby they refused to make peace with the Israelites.
Related Commentaries of Joshua 9:15
Joshua 9:15
15 And Joshua made peace with them, and made a league with them, to let them live: and the princes of the congregation sware unto them.