Matthew 14:1 - Dummelow's Commentary on the Bible

Herod the tetrarch] son of Herod the Great, received by his father's will the government (tetrarchy) of Galilee and Peræa. His first wife was the daughter of the Arabian prince Aretas, called in 2 Corinthians 11:32 king of Damascus. During a visit to his half-brother, Herod Philip (not the tetrarch), who lived as a private citizen in Rome, he became enamoured of his wife, Herodias, and persuaded her to leave her husband. He at once divorced his own wife, and married her. The marriage gave the greatest offence to devout Jews, for (1) it was unlawful to take a brother's wife after his death, much less while he was alive (Leviticus 18:16; Leviticus 20:21). The only exception was when the brother died without an heir (Deuteronomy 25:5-10). (2) Herodias was the niece of her new husband: see art. 'Dynasty of the Herods.'