Examples of the opposite are numerous throughout history: a technologically or militarily superior tribe or culture conquers a weaker one and proceeds to kill (or possibly enslave) the men and steal the women along with anything of value. They may or may not seek to control the territory. This is pretty much the standard pattern for such encounters.
What exceptions can be found? Where did a stronger group encounter a weaker one and act in a benign way, inherently respecting the culture, traditions and territorial rights of the weaker?
The Romans provide what could perhaps be considered minor examples of this - they apparently tended to avoid fully suppressing a conquered people, and made it possible for these to enjoy benefits of Roman civilization and even to become citizens.
What other examples come to mind?