According to Matthew, who should Joseph have thought was the ruler of Galilee, when he chose to move there instead of to Judea?
This is not a simple situation to suss out. (See part one.) We know that Archelaus was officially sole ruler of both Judea & Galilee until Caesar ruled differently, and the Emperor's ruling did not come until October or November of 4 BC (after Varus' war was wrapped up, Philip had sailed to Italy late in the season, and Caesar had deliberated some more). In other words, from late March of 4 BC until the end of the year, no one in Palestine had any reason to think Archelaus was not ruling Galilee, as well as Judea.
Now, Matthew and most of his readers certainly knew what happened later on - that Archelaus, Antipas & Philip returned from Rome early in 3 BC having each received only 1/3 of the Kingdom. Furthermore, Matthew draws a definite contrast here between Judea and Galilee, almost as if he's deliberately reminding us (perhaps only with his subtext) that we know why Galilee turned out to be safe, after all. But at face value, Matthew's use of "Judea" seems very odd. If Joseph departed the night Herod died, and (most likely) reached the outskirts of Judea just after Passover, then Joseph should have been fully aware that Galilee was also within the jurisdiction of Archelaus the horrible.
Once more, the question at hand is this: according to Matthew, who should Joseph have thought was the ruler of Galilee? The answer must be: Archelaus. That is, according to Matthew, Joseph was afraid to go into Judea AND he had not considered Galilee EITHER, because, of course, Archelaus was ruling there also.
This brings us to Matthew's point. The dream was necessary. Having been instructed ('warned' is a poor translation of χρηματισθεὶς here, and an editorial completely unnecessary, because Joseph was already afraid. The word simply means receiving a divine message, as from an oracle) in a dream, Joseph went to Galilee, a bit further from Archelaus' center of power, which was Judea.
By any reading, it should already have been apparent that Joseph would not have gone to Galilee without having that dream. But by reconstructing the details - and assuming that all Matthew's statements are entirely accurate - we see more clearly why Joseph needed the dream in the first place.
Of course, if anything Matthew said is non-factual, then the whole thing might be hooey. But if taken at face value, it all actually fits. And it fits very well. That's worth considering.