נחמיה, פרק י״ג, פסוק כ״ג

A solemn promise made by the people was ultimately broken. They violated an oath they had sworn earlier, specifically breaking their third major commitment: the strict ban on marrying foreign women [מלבי״ם]. The Jewish men actively brought these women into their own homes, settling them there with the deliberate intention of marriage [מצודת דוד].

The primary approach among commentators is that these women, who came from Ashdod, Ammon, and Moab, were active idol worshipers. Because of their idolatrous practices, they were strictly forbidden to the men in marriage.