When Herod Antipas divorced the Arabian princess who was his first wife, about 28/29 AD, she went home to her father, Aretas, King of Nabatea. Evidently, the divorce prompted both Antipas and Aretas to build up their military strength. Aretas' army defeated Antipas' army at Gamala in 36 - the first use of Nabatean force on record since 4 BC, and the only known instance of Galilee having an army!
Three years later, Herod Agrippa got his uncle Antipas exiled with three accusations before Caligula: an ongoing conspiracy with Parthia, a past conspiracy with Sejanus, and the possession of arms for 19,000 troops. The alleged conspiracies aren't necessarily credible, but Josephus tells us that Antipas couldn't deny having the cache of arms. Including defensive walls around Sephoris and Tiberias, this completes our knowledge of Galilee's military situation under Herod Antipas. Many questions remain.
Was the Herodian army illicit or sanctioned by Rome? Officially, Gamala had become Roman territory after Philip's death in 33/34. Given Antipas' high level of communication and Cooperation with Syria and Rome, it's hard to suppose the Tetrarch was trying to possess the Golan on the sly. His defeat there also suggests, quite strongly, that the force holding Gamala was much smaller in size than the cache could have equipped. Antipas' own appeal to Tiberius is the final clincher. Galilee must have been holding Gamala on Rome's behalf, and Galilee's army was fully sanctioned by the Empire.
We turn again to Agrippa's accusations. An alliance with Parthia is conceivable if Antipas' was recruited during the treaty he helped broker at the Euphrates in 36 (while his army was under attack). Antipas' continued dealings with Rome and Syria would have been good cover for treachery, and it still makes sense for Antipas to seek a crown from Caligula (in 39) after the Parthian King Artabanus died in 38.
If the cache was commissioned or supplied by Parthia beginning in 36, it explains why Gamala was taken so easily. Artabanus' death would explain why the cache remained unused, but it does not explain how Agrippa was able to discover all this. He only arrived in his territory in 38 and had little by way of immediate resources. Some Herodians could have defected to Agrippa, with intelligence, but that still doesn't make any of this true. We have no evidence to confirm or deny that Antipas was allied with Parthia.
We do, however, need to explain the cache of arms. That many units could have supplied Syria's four legions in most years or three at full strength. Either way, that's enough weaponry to challenge for total supremacy over the Region. By himself, Herod Antipas had neither the need nor the manpower for such a large force. Therefore, Parthia probably did commission the weapons, simply because no one else could have.
The only other candidate would seem to be Aelius Sejanus but I'm now prepared to discount that possibility. In 2008, I suggested that (1) Tiberius' keeping Governor Lamia away from Syria left the Eastern Legions with no central command from 23(?) to 31/32 AD, (2) Sejanus could have traded favors to allow Antipas' divorce in exchange for a military build up, and (3) the cache could have been kept on the sly simply to work around Tiberius, not to work against him. Sejanus was planning a regency of Tiberius' heir and had no need to plot against the Emperor himself until late in the game (31). He could have commissioned the cache benevolently, for Rome's protection, but I no longer think that he did.
It's still true that Agrippa's time with Antipas in Galilee (between 29 and 32 AD) gave him access to know things about Galilee, but if the cache was for Sejanus, why would it remain unused at Gamala, five years after Sejanus' death, and why would it still exist three more years after that? If it were Sejanus' cache, its purpose would be defunct in 31 and Antipas could have done any number of things with the materials. (Unless he was saving it to offer to Parthia - a third possibility is worth noting as strictly plausible, perhaps, but far less likely.)
In summary and conclusion, the cache itself makes the most sense if commissioned by the Parthians, and thus not existing until after Aretas' capture of Gamala. Any secret alliance between Antipas and Artabanus would have become defunct some time in 38, but Agrippa would have been back in the region in time to learn about it and report it (a year later) as if still current. The cache was the important accusation, and Caligula wasn't extremely astute at keeping up with developments abroad. In the end, it looks like Antipas probably did have some tentative alliance with Parthia, which must have been spurred by the growing instability in the East throughout that decade.
Altogether, this calls for another look at Agrippa's least credible accusation. The payoff here may bring us full circle, back to Antipas' divorce and the origins of his first army.
Neither Sejanus nor Antipas had any reason to plot against Tiberius, but that doesn't mean they had no dealings together at all. The years after Sejanus fell were dicey for anyone open to accusations regardless of whether they were true, and especially dicey if they were. Still, 39 was a long time after 31, and so Option One is that Agrippa made it up just for icing on the cake, to concoct a history of treachery, to secure Antipas' denunciation. This is probably at least partly true.
However, Option Two is that we recall Agrippa lived in Galilee from 29 to 32, the very years of Sejanus' political zenith and catastrophe. Yes, the joint treachery was probably concocted, but the best lies are built on a basis of truth, and Agrippa may well have been building on top of something he witnessed about Antipas' responses to news about Sejanus, during those years.
To review: Antipas' betrothal-agreement with Nabatea (c.1 AD) must have been smiled on by Augustus and he would have been unwise to end it (around 28/29) without some assurance or blessing from Rome. Josephus tells us the Tetrarch picked up his new wife Herodias (his niece) in Caesarea, took her with him to Rome, and returned as a couple before his Nabatean [soon-to-be-ex] wife made good her escape.
If Antipas had no more than this from Sejanus, we may wonder forever what Sejanus had from Antipas. But Herodias was well connected with Antonia, the last of three powerful matrons Sejanus could not control (after Livia died and Agrippina was banished). Antonia, in fact, is the one who eventually brought about Sejanus' downfall. Perhaps Sejanus only hoped Herodias would add persuasions in Antonia's ear. Or perhaps it was something else we know nothing about.
Whatever the case, Herod Antipas had reason to worry after 31 AD. Sejanus had not sent him an army, or commissioned a cache of arms in Galilee, but Sejanus most likely HAD assured Antipas of a peaceful divorce. With Tiberius now in his 70's, on Capri, with reports of political turmoil in Rome, with no Governor in Syria (Flaccus came in 32, the first in almost a decade, at which Agrippa left Galilee for Antioch), and now with no assurance of protection against Aretas' wrath - and all of that not to mention the Galilean backlash because of a beheaded prophet and the threatened insurgency for the new prophet drawing thousands to his side - the years 31 and 32 must have showed a very haggard side of Antipas from Agrippa's perspective. It was eight or nine months after Sejanus' fall that Agrippa decided to leave Galilee.
If the death of Sejanus caused at least that much stress to Antipas, and if his nephew and brother-in-law Agrippa noticed the Tetrarch's mood shift dramatically (more than others noticed, by his family connection), Agrippa might have been justified to merely assume the two rulers had been allies.
Again, we have no reason to think Antipas ever joined any plot against Tibeirus, but his divorce alone shows that he must have been allied with Sejanus to some slight degree. In 31 and 32, that one dealing was very much enough to be very concerned about, even for a client ruler abroad. On the one hand, Antipas had to worry about the heads rolling in Rome. On the other, Antipas had an Arabian ex-wife and her militant father biding their time, waiting for their opportunity. The divorce had reopened the need for vengeance on everything Herod the Great had done to Nabatea, also.
Whatever assurance let Antipas take Herodias back from Rome, it disappeared in 31 AD. In other words, Sejanus did not give Antipas an army, but Sejanus' death was probably what caused Antipas to begin gathering one.