Jamieson, Fausset and Brown on Mark 6:17

17. For Herod himself had sent forth, and laid hold upon John, and bound him in prison--in the castle of Machærus, near the southern extremity of Herod's dominions, and adjoining the Dead Sea [Josephus, Antiquities, 18.5,2].

for Herodias' sake--She was the granddaughter of Herod the Great.

his brother Philip's wife--and therefore the niece of both brothers. This Philip, however, was not the tetrarch of that name mentioned in Lu 3:1 (see on Lu 3:1), but one whose distinctive name was "Herod Philip," another son of Herod the Great--who was disinherited by his father. Herod Antipas' own wife was the daughter of Aretas, king of Arabia; but he prevailed on Herodias, his half-brother Philip's wife, to forsake her husband and live with him, on condition, says Josephus [Antiquities, 18.5,1], that he should put away his own wife. This involved him afterwards in war with Aretas, who totally defeated him and destroyed his army, from the effects of which he was never able to recover himself.