The Aztecs allowed some conquered peoples to govern themselves with relatively little interference to preserve resources while expanding their control. This process is called indirect rule: one group conquers another, but instead of spending a great deal of resources quelling resistance and establishing a government there, the conquerors allow the local governments to continue ruling; however, the conquerors (Aztecs included) often demand a tribute from these local governments. So while the local governments still rule, they are no longer sovereign and instead must answer to the conquerors if need be.
