Well that takes time, first the empire has to tell the media that there are weapons of mass destruction, and that the beast people are working on getting nuclear weapons. Then they have to do a false flag attack. This is going to be hard for the empire because they don't have Haliburton executives in their government.
How long has this tribe been here? Hundreds of years? And in that time, nobody ever explored the cave right next to their city? Even if they were starving and would welcome a bit of grilled spider? Not even for a dare?