Jephthah first tried to solve the issue through diplomacy instead of using military force (compare Deuteronomy 20:10–11).
Unlike the nomadic and raiding Midianites from Gideon's time, the Ammonites were a settled tribe. Their land was east of Israel's territory in Transjordan (east of the Jordan River), with Rabbah as their capital. The Ammonites, like the Moabites (both who were descended from Lot) and the Edomites (descendants of Esau), were to be left alone because the Lord assigned their territories to them (Deuteronomy 2:16–23).
Contrary to the Ammonite king's claim, the Amorites, not the Ammonites or Moabites, lived between the Arnon and Jabbok rivers during the conquest. When Sihon refused to let Israel pass, the Lord gave the land of the Amorite kings Sihon and Og to Israel. The Ammonite king had no valid reason to argue with Israel.