Answer:
Usually a civilization comes to power, rises(on the back of a charismatic Leader or leader's usually, Then succumbs to Decadence and falls because the NEXT world power pushes them of their pillar of power IE:Persia & Greece, Greece and Macedonia, Carthage and Rome ,Rome and the Goths (Rome's fall was and exception because though it was crushed through weakness,the powers Defeating it(Goths,Visigoths) were unable to fill the void Rome left)England and The United States is probably the best example of this;the country gets too Rich and lazy to continue being a military superpower and falls.
Hope this helps
A3