The Fundamental Laws of the Star Imperium recognized the right of individual worlds to have their own local military and police forces collectively called Planetary Security Forces. This right continued under the laws of the United Planets.
On occasion, a rebellious planet desired to break away from the Imperium or the United Planets, and the central regime on Alfheim or the Moff of the local Stellar sector called in the Intergalactic Navy and Stormtroopers to deal with such perceived treason.