The king of the Ammonites wanted the right to return to lands that Israel had won in battle, some 300 years earlier.
Judges 11:13 When the Israelites came out of Egypt, they stole my land from the Arnon River to the Jabbok River and all the way to the Jordan. Now then, give back the land peaceably.
He didn’t stop to remember that he’d won the land in battle before that from the Amorites and Moabites.
Jephthah said no: this king was dreaming if he thought Israel would just hand over land without a fight. Israel had won the battles fair and square, and Jephthah reminded this king of the unwanted and unexpected battles the Israelites had to fight just to get through the lands this king now wanted:
Judges 11:23-25 So you see, it was the Lord, the God of Israel, who took away the land from the Amorites and gave it to Israel. Why, then, should we give it back to you? You keep whatever your god Chemosh gives you, and we will keep whatever the Lord our God gives us. Are you any better than Balak son of Zippor, king of Moab? Did he try to make a case against Israel for disputed land? Did he go to war against them?
Jephthah’s advice was to let God decide:
Judges 11:27 Therefore, I have not sinned against you. Rather, you have wronged me by attacking me. Let the Lord, who is judge, decide today which of us is right—Israel or Ammon.
It was a winner-takes-all approach:
Judges 11:32-33 So Jephthah led his army against the Ammonites, and the Lord gave him victory. He crushed the Ammonites, devastating about twenty towns from Aroer to an area near Minnith and as far away as Abel-keramim. In this way Israel defeated the Ammonites.