Albert Barnes Commentary Ruth 1:4

"And they took them wives of the women of Moab; the name of the one was Orpah, and the name of the other Ruth: and they dwelt there about ten years." — Ruth 1:4 (ASV)

Marriages of Israelites with women of Ammon or Moab are nowhere in the Law expressly forbidden, as were marriages with the women of Canaan (Deuteronomy 7:1–3). In the days of Nehemiah the special law (Deuteronomy 23:3–6) was interpreted as forbidding them, and as excluding the children of such marriages from the congregation of Israel (Nehemiah 13:1–3). Probably the marriages of Mahlon and Chilion would be justified by necessity, living as they were in a foreign land. Ruth was the wife of the older brother, Mahlon (Ruth 4:10).