Why did Yiftach mention Mo'av? Melech Amon was complaining about taking Amon's land!
Radak: Melech Amon demanded from Arnon until Yabok. Yiftach answered that Mo'av used to own it, and Sichon took it, and Yisrael took from Sichon. Yisrael did not take from Amon or Mo'av.
Refer to 11:15:2:1.
Malbim (Shmuel 1:11:9): Amon and Mo'av were united at the time.
Why did Melech Amon complain about taking from Arnon until Yabok? It was of Mo'av, for Arnon was the border between Mo'av and the Emori!
Malbim (13): They demanded that what Yisrael took from Sichon be returned to its previous owner. It seems that Amon ruled over Mo'av when Sichon took it. Also, Sichon took also Amon. It says (Yehoshua (12:2, 13:25) "me'Aro'er? Gevul Bnei Amon" and "ha'Gevul Ya'azer? va'Chetzi Eretz Bnei Amon."
Refer to 11:15:1:3.
