Those ensnared in adultery often become blind to common sense, including the warnings of those close to them. Antipas, son of Herod the Great (2:1) and a Samaritan mother, hence Archelaus's full brother (2:22), had functioned as tetrarch over Galilee and Perea since about 4 B.C. He had entered into a politically prudent marriage with a Nabatean princess, perhaps seeking to secure further loyalty from Nabatean subjects within his territory of Perea (Kraeling 1951:89).
But when Antipas divorced his first wife to take his brother's wife, he violated not only Jesus' teaching on the moral indissolubility of marriage (5:31-32) but also the Mosaic law concerning incest (Lev 18:16; 20:21). John thus publicly reproached a public example of immorality. But what John viewed in moral terms Antipas undoubtedly saw in political terms as well (compare Jos. Ant. 18.118; Kraeling 1951:85, 90-91, 143-45). Antipas's plans to divorce his first wife had provoked trouble with her father, the powerful Nabatean king Aretas (on whom see 2 Cor 11:32-33). This trouble ultimately led to war and public humiliation for Antipas (Jos. Ant. 18.113-14, 124-25). That many Nabateans in Perea presumably remained loyal to Aretas further extended the political implications of Herod's affair. A prophet harping on the tetrarch's misbehavior was therefore politically dangerous.
Christians today who take a stand against abortion, exploitation of the poor or racism may be taking a moral stand, but in our polarized society many will read such a stand as politically partisan even when we do not intend it in such terms. The major difference at this point is that John's society did not recognize freedom of speech; publicly denouncing a ruler's character was essentially suicidal. Israel had a long-standing tradition exempting prophets from severe punishment for their speech-a rule that only the most vicious rulers broke. Unfortunately for John, Antipas proved to be such a ruler.