Jephthah is from Gilead. Gilead belonged to the half-tribe of Manasseh. Israel has lost its way at this point. They have no leader. They are doing what’s right in their own eyes.
The Ammonites have come to make war on Gilead, so they send for their half-brother, Jephthah, who they know is a mighty warrior. Before they had rejected him for being just a half-brother, but now they agree to make him their leader if he fights for them.
They are appointing their own leaders instead of letting God save them and letting God raise up a leader if He so chooses. There’s no mention of the inhabitants of Gilead ever calling on the Lord for help. They rely on the strong man.
The strong man must realize his limitations because he calls on the Lord, sort of. He doesn’t admit he needs help or ask for help. He makes a vow. He vows to sacrifice whatever comes out of his house to greet him if he wins. What was he picturing? His dog? I have no idea.
It was a foolish thing to say. It was foolish to think he could buy God’s favor. He didn’t win because of his vow. He won because God had promised the land to Israel and was not yet ready to take it from them.
We see the Lord’s justice work out on many levels. The Ammonites accuse Israel of unjustly having their land. The Ammonites get a lesson on how they were the ones who refused to help Israel when they asked for it and prompted the fighting that ended with Israel taking their land. Israel had won the land in battle. God defends Israel justly.
Jephthah doesn’t act justly. He unjustly kills his daughter because of his fear of failure. God would have released him from his oath. He would have accepted a redemption price as His law allows. Her death was not on the Lord’s hands. He is a just God.