After besieging Gilead the Ammonites demanded their land which they claimed the Israelis took from them during their exodus from Egypt to Canaan.
Judges 11:4-13 KJV
4 And it came to pass in process of time, that the children of Ammon made war against Israel.12 And Jephthah sent messengers unto the king of the children of Ammon, saying, What hast thou to do with me, that thou art come against me to fight in my land? 13 And the king of the children of Ammon answered unto the messengers of Jephthah, Because Israel took away my land, when they came up out of Egypt, from Arnon even unto Jabbok, and unto Jordan: now therefore restore those lands again peaceably.
God had clearly instructed the Israelites not to take any land from the Ammonites
Deutoronomy 2:19-36 KJV
19 And when thou comest nigh over against the children of Ammon, distress them not, nor meddle with them: for I will not give thee of the land of the children of Ammon any possession; because I have given it unto the children of Lot for a possession.
Deutoronomy 2:32 KJV
32 Then Sihon came out against us, he and all his people, to fight at Jahaz. 36 From Aroer, which is by the brink of the river of Arnon, and from the city that is by the river, even unto Gilead, there was not one city too strong for us: the LORD our God delivered all unto us:
The land that the Israelites took from Og & Sihon the ammorites stretched from the Arnon river to Jabbok.Arnon river was the border of Moab between Moab & Amorites whilst the Jabbok river was the border of the Ammonites.It is clear that Israelites attacked the Amorites as far as the border of the Ammonites which was heavily fortified.
It is only recorded that the Amorites took land from the Moabites( Numbers 21:27-30).Nothing is said of them having taken it from the Ammonites.
Could it be Sihon & Og the Amorites kings had taken the land from the Ammonites well before?