I am repeatedly hearing a theory that empires, and polities in general, have a very limited timespan. Their life follows a predetermined pattern: first, they are established, then they expand and grow in power, then they reach their golden age, when they are at the summit of their wealth, power and influence, though expansion stops; then they will slowly decline, never to return to their golden age, and finally, they must fall. During their decline there may be temporary restorations, however...
