Pre-clarification: In this article, a 'common enemy' does not always mean a corrupt individual or human organization. The term is used in a broader sense that includes, but is not limited to, ideas that one doesn't agree with, impending natural disasters, after-effects of recent natural disasters, and (if possible) even an invasion by aliens.
There seems to be something strange about human beings in general - they always put aside their differences and band together to fight a common enemy. But take that common enemy away, and they will pick the differences back up and fight among themselves.
And in this "fight among themselves", a new "common enemy" is created based on one difference or a set of related differences - and those on one side of this particular difference would put aside all of their other differences and band together to fight the other side of the particular difference.
If you assume that in each fight one side always loses within a reasonable amount of fighting time, and a compromise will never be reached, eventually human beings would fight their way into destroying themselves. Why? Because each one of us is different from the rest - even in ever-so-slightly ways.
Thankfully, we are also wired for co-operation when our survival is at stake. That's kind of the problem too: we only co-operate with other humans, putting aside our differences, if our survival - or something of such value - is at stake. Simply put, the inability to survive becomes the "common enemy" in this case, causing humans to band together and fight for their survival.
The problem with large stable populations of humans is that their survival is no longer perceived to be at stake, and so they have not much need to co-operate other than what's just enough to smooth over the everyday life. This gives them no common enemy to band together - and that makes them susceptible to picking up differences among themselves and starting a competitive argument. Once such argument reaches popularity, sides are formed and a new "common enemy" is created for each side opposing the other.
The most elegant solution, or perhaps the only one, to this problem - in my opinion - would be to educate human beings and make ourselves understand that the survival of our species is always at stake no matter how much stability we may perceive. Our survival is threatened by ourselves, and hence we should maintain co-operation at the highest levels to fight the real common enemy - our tendency to compete over differences.