In these days, possession was nine tenths of the law! A decree from the king (hundreds of miles away) didn’t mean too much to the men out on the frontier. The Jews were a minority. Even though they were scared, it is admirable that the Jews immediately acted on their fear, building the altar. It would have been easy to set up a low profile. For more information, read the book of Nehemiah, where we see an intense battle over reclaiming this territory.