What does Judges 11:25 mean?

Jephthah now arrives at a third argument countering the king of Ammon (Judges 11:13). This enemy ruler is entirely wrong in thinking his people have any right to the land of Gilead. Jephthah's historical argument was simply that the Ammonites never possessed the land in the first place (Judges 11:14–22). His theological argument is that the Lord God of Israel gave the land to the Israelites, meaning it was His divine will for Israel to possess it (Judges 11:23–24).

This final point is that other, better kings have not tried to take Gilead from Israel. Balak was the king of the nearby Moabites when Israel came into possession of the land of Gilead (Numbers 22:1–3). Balak was terrified of Israel, and he unsuccessfully tried to get the prophet Balaam to curse Israel on behalf of the Moabites (Numbers 22—24). King Balak, however, did not go to war against Israel for the bordering land of Gilead. That is Jephthah's point to the current king of Ammon: even famous King Balak did not attempt what you're trying to do.