Yes. Civilisation can slip into barbarism at the drop of a hat, and history is littered with examples of it, unfortunately. And this is exactly why people consider the Constitutional restraints placed on the Government's authority over the individual so important, because they are our only defences against a huge monopolistic power with near unlimited resources at its disposal.
They are much, much more important than Democracy.
Also, these periods of barbarism and anarchy are usually preceded by periods of complacency (no, that could never happen, peace and prosperity are things that last forever
), the kind of complacency that grows in a country which has only really been at war on its own soil a single time, and that was against itself. This is also why the spirit of liberty is strongest in a nation in the years directly following the revolution, when people's energies are greatest (i would say the spirit of 76 was the only thing that held North and South together in the many years before the North subsumed it).