From a piece by Nicholas D. Kristof in today's New York Times:
We might recall what happened to ancient Athens, perhaps the greatest flowering of civilization. In just three generations, one small city--by today's standards, anyway--nurtured democracy, became a superpower and produced some of the greatest artists, writers, philosophers and historians the world has ever known.
Yet Athens became too full of itself. It forgot to apply its humanity beyond its own borders, it bullied its neighbors, and it scoffed at the rising anti-Athenianism. To outsiders, it came to epitomize not democracy, but arrogance. The great humanists of the ancient world could be bafflingly inhumane abroad, as at Melos, the My Lai of its day.
Athens's overweening military intervention abroad antagonized and alarmed its neighbors, eventually leading to its defeat in the Peloponnesian War. It's not so much that Athens was defeated--it betrayed its own wonderful values, alienated its neighbors and destroyed itself.