Successful civilisations throughout history have grown rapidly, appearing unstoppable. Some (like Rome, and Han China) existed stably for hundreds of years. Others, like Alexander the Great‘s Asian Empire did not. But in every kingdom, empire and state that has ever existed (excluding the few surviving today, which are probably not as eternal as we’d like to think), something has eventually changed. Once great empires are defeated by a smaller rival (e.g. Sassanid Persia), and others crumble all by themself. Changing fortunes are one of the clearest signals in history.