Traditionally there have been a number of factors that have held human societies together. A common ethnicity, a shared culture and history, religion, or sometimes merely loyalty to the ruling dynasty.
In the late 18th century there were two spectacular attempts to find an alternative means of national cohesion, based on shared values and political beliefs. Those two experiments were the French Revolution and the American Revolution.
The French Revolution tried to substitute slogans - liberté, égalité, fraternité - for traditional loyalties to the Crown. The results were not pretty. The French at least had the advantages of still having a more or less ethnically homogenous state and a shared culture and history. The history of France since the Revolution has however been one of extreme political instability.
The American Revolution was even bolder. The original thirteen colonies were reasonably ethnically homogenous but although they did have a shared culture and history it was one that had been transplanted to another continent. Traditional ties to the land itself were obviously much weaker. The United State was a daring attempt to base a nation state on shared political and philosophical beliefs.
It sounded like a great idea at the time. The trouble with political and philosophical beliefs is that they are inherently unstable. It is also rather rare to find any society in which every member of society genuinely shares the same beliefs. 239 years after the Declaration of Independence it is clearly absurd to believe that there are any values at all that are genuinely shared by all Americans. And the ethnic homogeneity and the shared culture and history have gone by the board.
When a society is based on the concept of all citizens believing more or less the same thing finds itself transformed into a society bitterly divided along political lines, and when that society has no shared culture or religion to bind it together, what is there left to prevent complete social dissolution?
Societies based on political and economic principles, such as the United States or the Soviet Union, sound terrific and impressively rational in theory. In practice there’s a lot to be said for the old-fashioned notion of emotional loyalty to King and Country.