Jephthah is an interestingly tragic man. As an illegitimate "love child", he was outcast by his family and community. He became an outlaw and scoundrel, but he seems pretty level headed. When the people of Gilead had no one to turn to, they asked him to return. They had nothing to lose if he died. Jephthah is a believer and trusts the Lord, making his tragic vow. He has great success against the Ammonites, " with a very great slaughter" of 20 cities (maybe 200,000 dead?). His faith leads him to fulfill his vow and kill his only daughter as a burnt offering. Then his own tribe, the Ephraimites start a civil war with him. At least 42,000 Israelites from the tribe of Ephraim died.
He was able to cast off the oppression of the Ammonites, but we see the start of a power struggle within and among the 12 tribes