Adam Clarke on Deuteronomy 2:37

Only unto the land of the children of Ammon thou camest not - God gave them their commission; and those only were to be cut off, the cup of whose iniquity was full. Though the Moabites and Ammonites were thus spared, they requited good with evil, for they fought against the Israelites, and cast them out of their possessions, Jdg 11:4, Jdg 11:5; 2Chr 20:1, etc., and committed the most shocking cruelties; see Amo 1:13. Hence God enacted a law, that none of these people should enter into the congregation of the Lord even to their tenth generation: see Deu 23:3-6.