Darwinism in action. Isn't this forum pro-theory of evolution and against imperialism?
Actually, letting the weak die and selecting only the fittest is almost diametrically opposed to the very idea of civilization. Our goal as civilized animals is to make selection an outdated part of evolution. Cure the sick, help the handicap, preserve the old, protect the young.
We don't let people die to diseases in order to strengthen mankind. We take the helm and exterminate pathogens.
Either things are reorganized such that they gain more local autonomy granted effectively losing the top down control or they break off. In either case they typically do better because of it. Even with problems in the aftermath they typically rise further - India for instance.