Albert Barnes Commentary Deuteronomy 23:3-5

"An Ammonite or a Moabite shall not enter into the assembly of Jehovah; even to the tenth generation shall none belonging to them enter into the assembly of Jehovah for ever: because they met you not with bread and with water in the way, when ye came forth out of Egypt, and because they hired against thee Balaam the son of Beor from Pethor of Mesopotamia, to curse thee. Nevertheless Jehovah thy God would not hearken unto Balaam; but Jehovah thy God turned the curse into a blessing unto thee, because Jehovah thy God loved thee." — Deuteronomy 23:3-5 (ASV)

This law forbids only the naturalization of those against whom it is directed. It does not forbid their dwelling in the land and seems to refer to nations rather than to individuals. It was not understood, in any case, to prohibit marriage with a Moabitess (Ruth 4:13). Ruth and her sister, however, were doubtless proselytes (Deuteronomy 23:4).

Compare this reference: the Moabites and the Ammonites are to be regarded as clans of the same stock rather than as two independent nations, and as acting together (compare 2 Chronicles 20:1).