The end of the Roman Empire in the west came as more and more tribal groups settled inside its borders and its own citizens became increasingly reluctant to defend an empire that was divided into super-wealthy landed gentry and almost serf-like peasantry, causing military roles to be subcontracted out to the barbarian invaders who became, in essence, the Roman army.
The end of the British Empire came when Britannia had been physically bled white by one global conflict and then fiscally bled white by a second and, parting with its colonial possessions, the once-globe-spanning empire shrank back down to its pre-Elizabethan roots as a rainy tourist trap with good beer.
The end of the American Empire came when it was decreed that its fragile children were such precious pearls that they couldn't be exposed to the dangers of drawstrings on clothing. Seriously. This is the inevitable end result of bicycle helmets and rubber playground matting: Your average newborn these days is doomed to a childhood that makes Tod Lubitch look like Johnny Knoxville. No wonder they take up BASE jumping as soon as they get to college.
In the brief span of my lifetime, we have gone from a nation where kids could order a .22 in the mail (with their parents' permission) and go to Sears and buy a kit for casting their own lead toy soldiers to a nation where they are protected from dangerous clothing and would be expelled from school for drawing a picture of a toy soldier.
Sic transit gloria mundi.