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.

Wesley's Notes

9:15 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, 2Sam 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, Jos 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.

Matthew Henry's Concise Commentary

9:14-21 The Israelites, having examined the provisions of the Gibeonites, hastily concluded that they confirmed their account. We make more haste than good speed, when we stay not to take God with us, and do not consult him by the word and prayer. The fraud was soon found out. A lying tongue is but for a moment. Had the oath been in itself unlawful, it would not have been binding; for no obligation can render it our duty to commit a sin. But it was not unlawful to spare the Canaanites who submitted, and left idolatry, desiring only that their lives might be spared. A citizen of Zion swears to his own hurt, and changes not, Ps 15:4. Joshua and the princes, when they found that they had been deceived, did not apply to Eleazar the high priest to be freed from their engagement, much less did they pretend that no faith is to be kept with those to whom they had sworn. Let this convince us how we ought to keep our promises, and make good our bargains; and what conscience we ought to make of our words.