Charles Ellicott Commentary Numbers 21:24

"And Israel smote him with the edge of the sword, and possessed his land from the Arnon unto the Jabbok, even unto the children of Ammon; for the border of the children of Ammon was strong." — Numbers 21:24 (ASV)

For the border of the children of Ammon was strong.— These words assign the reason why the conquests of the Amorites were arrested, not why the children of Israel did not take possession of the land of the Ammonites, with whom they were forbidden to meddle, and whose land they were not to occupy. (See Deuteronomy 2:19.)