Numbers 21:2
And Israel vowed a vow unto the LORD, and said, If thou wilt indeed deliver this people into my hand, then I will utterly destroy their cities. And Israel vowed a vow unto the LORD, and said, If thou wilt indeed deliver this people into my hand, then I will utterly destroy their cities.
The vow was made after the Canaanites under King Arad attacked Israel, defeated them in battle, and carried away some Israelites as prisoners. In that solemn vow Israel promised the Lord that if He would deliver this people into their hand, they would utterly destroy their enemies and their cities. The vow also asked God to go with Israel to battle and promised faithfulness in following God in return. God heard and answered their prayer with divine aid, going out with their armies to battle, so Israel routed and overcame their enemies and utterly destroyed them and their cities.
This victory over King Arad and the Canaanites gave the surrounding nations further evidence of God's power and presence with Israel. That recent victory gave the people hope as they faced the Amorites, though there was no deeper humility or firmer trust in God to be seen.