The answer is: Ammonites

Judges 11:5

The story of Jephthah is one of the most intriguing and complex in the Book of Judges, weaving themes of leadership, sacrifice, and the challenges of faith. Jephthah, the son of a harlot, not only faced rejection from his own family but was also called to lead Israel during a time of crisis against the Ammonites, who were pressing on Israel's borders (Judges 11:5).

What's fascinating about Jephthah's narrative is his use of diplomacy with the Ammonite king before conflict erupted. In Judges 11:12-28, Jephthah attempts to reason with his adversaries, reflecting a common biblical motif of conflict resolution that often involves both negotiation and divine backing. Unfortunately, the king of Ammon is unmoved. This conflict reveals the often turbulent relationships between Israelite tribes and surrounding nations.

Interestingly, Jephthah is well-known for a rash vow he made to God, promising to sacrifice the first thing that came out of his house if he defeated the Ammonite forces. Many scholars debate the implications of this vow, with some arguing it reflects the tensions between ethical obligations and strict adherence to oaths. This serves as a reminder of the complexities faced by individuals navigating their faith amid dire circumstances.

Ultimately, Jephthah's leadership—posthumously colored by his tragic vow—reflects both the burdens and expectations placed on leaders. It’s a powerful example of how one's actions can lead to unforeseen and often tragic consequences, making readers ponder the weight of their decisions and convictions.