Only unto the land of the children of Ammon you came not, nor unto any place of the river Jabbok, nor unto the cities in the mountains, nor unto whatsoever the LORD our God forbade us.
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George Leo Haydock
Jeboc, towards its source; for both sides of the torrent were taken by the Hebrews from the kings Sehon and Og, who had already driven the Ammonites farther into the mountains, on the east. (Haydock)
When these demanded the conquered country to be restored to them, Jephte showed that their claim was inadmissible, Judges xi. 13. (Calmet)
They had lost possession when the Hebrews came; and, as God had authorized the latter to take the land from those kings, without enquiring who were the former proprietors, the Ammonites, who had themselves expelled the Zomzommim, came with a very bad grace to assert their title, after a lapse of near 300 years. God only forbids the Hebrews to molest the actual dominions of the sons of Ammon, Moab, and Esau, in consideration of their fathers, to whom they were so nearly related. (Haydock)