The Bible says when the Israelites were conquering some nations on their way to the promised land, they stopped at the fortified border of the Ammonites. God told them to leave the Ammonites alone, and the Israelites obeyed, and kept away from the land of the Ammonites.
Jephthah’s statement was in response to an accusation from the king of the Ammonites, who demanded that the Israelites give back the land they had stolen from his country. Why would he claim that Israel had taken land from the Ammonites? Well, according to the Bible, it was true. The Israelite tribe of Gad owned half of the Ammonite country.