Nehemiah 13:25
And I contended with them, and cursed them, and smote certain of them, and plucked off their hair, and made them swear by God, saying, Ye shall not give your daughters unto their sons, nor take their daughters unto your sons, or for yourselves.
This verse belongs to Nehemiah's reform after returning from Persia to Jerusalem to correct evils that had crept in during his absence. It was part of instructing Jerusalem in the law of God and enforcing obedience, the same reform in which the Levites were directed to guard the gates of the temple courtyard against traders and burdens. He confronted Jews, including men in high position and rulers who should have given counsel and safe example, who had married wives of Ashdod, Ammon, and Moab.
The oath not to give daughters nor take daughters concerns intermarriage with idolaters and bitter enemies such as Tobiah the Ammonite, forbidding Israelites to give their daughters to foreign men or take foreign women for themselves or their sons. Such intermarriage produced unhallowed friendship and alliance, gave idolaters a foothold in the city and even contaminated the precincts of the temple, and defied God's declaration through Moses that Ammonites and Moabites should be forever shut out from His congregation, showing contempt for God and His truth. The children of these marriages spoke half in the speech of Ashdod and could not speak the Jews' language, causing great confusion in Israel and threatening ruin if the evil continued.
Nehemiah reasoned earnestly with the wrongdoers, pointing to Solomon, to whom God gave great wisdom with no king like him among all nations, yet whose idolatrous women turned his heart from God and whose example corrupted Israel, and asking whether Israel should hearken to do all this great evil. He set before them God's commands and threatenings and the fearful judgments visited on Israel in the past for this very sin, until their consciences were aroused. The prohibition applied to existing marriages as well as future ones, so marriages already formed were dissolved and ties to idolaters were broken to prevent Israel from losing its distinct, holy character. A work of reformation was begun that turned away God's threatened anger and brought His approval and blessing.