Birthright: The Book of Man
Mike ResnickNot all chapters deal with humanity's treatment of aliens; some also cover the "internal" politics that result in a development of the growing human empire from a democracy to a monarchy. But the biggest theme is undeniably the search for the elusive quality that allows humanity to overcome all opposition and manage the unique feat of conquering the entire galaxy. It is never clearly defined but manifests perhaps most succinctly when it also results in the failure of an attempt to cross the void between galaxies.
Then, after there is no more room for conquest, the only way left is down: internal struggles as well as deep-seated resentment of aliens result in a decline of human power that takes nearly as long as the rise, but is described far less extensively. Somehow, despite whatever enabled humans to achieve total power, they were unable to keep it.