John Gill Bible Commentary

And the king of the children of Ammon answered unto the messengers of Jephthah,.... Who this king of Ammon was is not said, however he returned an answer to Jephthah's messengers, which they brought to him, and it was to this purpose; that the reason of his invading the land, and bringing war into it, was: because Israel took away my land when they came out of Egypt; not as soon as they came out of Egypt, for it was thirty nine years afterwards, and upwards, even a little before they entered into the land of Canaan; and the land they took was not theirs, but in the possession of Sihon and Og, kings of the Amorites; though indeed, before their conquest of it, it had been in the hands of the Moabites and Ammonites, and who being confederates, or subjects of the same king, is here claimed by the king of the children of Ammon: from Arnon even unto Jabbok, and unto Jordan; the river Arnon was the border between Moab and the Amorites, and the river Jabbok was the border of the children of Ammon, Num 21:13, the one was to the south of the country claimed, and the other to the north and to the west, which was Jordan, and the wilderness to the east, Jdg 11:22, now therefore restore these lands again peaceably; this is demanded or proposed as terms or conditions of peace, and what would prevent a war, and nothing short of this would do it.