11:15-18
Jephthah gives the Ammonite king a history lesson. He recounts events which were recorded in Deuteronomy 2:9 and 2:19 in particular. Israel had not been on a war footing. They had not taken the land of Moab nor of the Ammonites. Even when Edom refused them safe passage they did not attack, but took a different route (Num 20). Instead they skirted the lands of Edom and Moab, not entering them. It is useful to know one's history. This is still true today. There is a lot of nonsense talked about Israel, there is also a lack of knowledge with regard to the worldwide history of slavery and colonisation.
11:19-22
Having dealt with cases where Israel accepted the refusal to allow them through their country, Jephthah now turns to cases where Israel did engage in battle. In these cases the recalcitrant nation started the war. Israel won the war, and took the land from the Amorites, so it was not taken from the Ammonites.