What did Rome get from approving Antipas' re-wifing? Aside from Antonia's patronage, or Antipas' improving tax base in Galilee, what else could possibly have swung Tiberius and/or Sejanus into favoring the switch? (Series so far: 1, 2, 3, 4, 5)
It could be that Aretas was ripe for a demotion in status to begin with. The Nabatean King hadn't won much favor from Augustus, after his crowning in 9 BC, and so far as we know Aretas had done nothing to please Rome since the marriage treaty with Antipas. What we do know is that Aretas spent a great deal of his resources on improving his own kingdom, and yet unlike his Judean counterpart, Herod the Great, we have no record of Antipas contributing benefactions to the Greek-Roman oikoumene at large. If things were as they appear to have been, such selfish isolationism cannot have won any favor at Rome. More likely, it was quite to the contrary.
By contrast, then, one of Herodias' chief advantages over the Nabatean princess - in Rome's eyes - was Herodias' affinity for Latin culture and for Rome itself.
At the start of this series, we noted how important it was to Augustus (and thus to Tiberius, because Tiberius' positions in foreign policy were uniformly conservative) that foreign rulers be raised and indoctrinated at Rome. It may not have taken Antonia to point out to Tiberius (or Sejanus) how perfectly Herodias' upbringing met Rome's preferred qualifications. She was of royal, Herodian blood. Her husband-to-be, Antipas, had spent several years in education at Rome, but she herself had been raised there from infancy, within the Imperial household!
In start contrast to the princess of Nabatea, whose father's Kingship was only approved for a lack of alternatives, Herodias was a woman whose pedigree was contributive. In the long road towards (hoped for) Romanization, which was looking especially long in the East, Rome needed to provide backwater regions such as Galilee with as much positive acculturation as it possibly could. Installing Herodias with Antipas was like installing the mint. Every developmental assistance encouraged stability.
Although no one saw this marriage as anything so anachronistic as a true partnership of joint rulers, there was definitely some gain for the Empire in giving Herodias to Antipas. In turn, Antonia and Tiberius (and/or Sejanus... and all their advisors) would have recognized in Herodias another positive way to contribute direct Romanizing influence on a client King in the East.
That - in addition to whatever other advantages Antipas or Antonia may have offered - is why Rome made Herodias Queen - essentially, if not officially or titularly - of Herod Antipas' Galilee.
To be Concluded...
Read the Whole Series: Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Conclusion