The answer is: Ammonites

Judges 11:5

The story of Jephthah and the Ammonites is one of the captivating narratives found in the Book of Judges. Jephthah was not your typical leader; he was the son of a prostitute and grew up in a rather challenging environment. It's interesting how this speaks to the broader theme throughout Judges: God's choice of unexpected deliverers. Jephthah rises from being outcast to a valiant hero who combats the Ammonites, adding a rich layer of redemption to his character.

Now, the Ammonites themselves had a troubled relationship with Israel, stemming from historical tensions and territorial disputes as described in earlier biblical texts. They were descendants of Lot, Abraham's nephew, and their animosity grew out of longstanding conflicts between Israel and neighboring nations, reflecting a cycle of struggle found throughout the period of the Judges.

Jephthah famously makes a rash vow before battling the Ammonites, pledging to sacrifice whatever comes out of his house first upon his return. This tragically ends up being his daughter, infusing the account with sorrow and raising intriguing discussions about vows and their implications in human life. The story, steeped in drama and moral complexity, echoes throughout time, prompting believers to examine the commitments they make and how they align with their actions—or regrettably, sometimes don’t. The tale serves as a stark reminder of the complexities of leadership and the unfathomable elements of the human spirit.