Uller Uprising by H. Beam Piper, Science Fiction, Adventure

"ZNIDD SUDDABIT!"
So the Ulleran challenge begins, with the rantings of a prophet and a seemingly incidental street riot. Only when a dose of poison lands in the governor-general's whiskey does it become clear that the "geeks" have had it up to their double-lidded eyeballs with the imperialist Terran Federation's Chartered Uller Company. Then, overnight, war is everywhere.
How it will end is in the (merely) two Terran hands of the new governor-general, a man shrewd enough to know that "it is easier to banish a habit of thought than a piece of knowledge." The problem is, the particular piece of knowledge he needs hasn't been used in 450 years. . . .