Because it would be such an incredible coincidence if two great powers were exactly evenly matched in the first place, and even more if they stayed exactly evenly matched for a long period.

• Hegemonic wars start because different great powers, seeing the fading of the importance of what used to be dominant power resources, have different views of where they stand relative to the other great powers.
  
• The hegemonic war settles which power resources really are the dominant combination.
  
• Winning the hegemonic conflict puts the hegemon in a position to work on reforming the previous world order, normally in its interests, and that reinforces the hegemon's position.
  
• But ongoing social and technological change ensures that no set of dominant power resources is permanent, so sooner or later the benefit of the power resources that lead to the dominance of the hegemon begins to fade.
  
• It is one of those powers that are gaining under the new conditions that is most likely to pass the incumbent hegemonic power to become a new hegemon, but of course its always possible that the existing hegemon develops the power resources that end up being critical in the next cycle.

So like cycles in economics, its a standing wave ... not driven by exogenous shocks, or by some kind of interplay of ideal underlying forces, but a self-perpetuating dynamic system.

Its most closely associated with the rise of Europe (and then a former European settler colony) from LDC backwater status pre-1500 to world dominance. There is of course much dispute (that is much of the relevance in the fight over the dash and capitalization between world-system theory and World System Theory) whether that's an extension of an older pattern or a pattern that was launched in the second half of the previous millenium. For Wallerstein, it is a pattern that is typical of the European world-system, which grew from a region of the world in 1500 to nearly encompass the world by the 1800's, others have other views on that.

And it is certainly not seen as the only possible historical pattern ... for example, if a hegemon continues rising in power to the point that it establishes an Empire, then the dynamic of rival powers vying for their position in a system of state does not apply, and that would break the standing wave.

Indeed, one of the concerns of both world-system and World System theorists is how to disrupt the perpetuation of the pattern so that we can avoid having "the next" hegemonic war ... which in an age of nuclear and biological weapons is a terrible thing to contemplate.


I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.