Historically, civilizations and empires that sought to expand their power globally nearly always seemed to crumble from such a high point of power and dominance down to almost nothing. Examples being the Roman empire, Ottoman Empire, Germany (WWII), Louis XIV's absolutist government, and Great Britain. Why is it that these nations seemed to always be reduced in strength by either economic or political problems within their own territory and not necessarily an outside invading force?
When an Empire or great power is at their peak, there are no outside invading forces that could take them on.
Take Britain for example, when it was at the peak of it's power, the world was in 'Pax Britannica', a period of relative peace because no one could challenge Britain militarily and didnt want to further weaken themselves (comparatively) by fighting others. When the Royal Navy is bigger than the next two biggest navies combined, it's hard to fight.
When the empire/civilisation is that strong, it is pwoerful enough that beating it would take more resources than anyone else had/wanted to spend on the war, which leads to more indirect methods being the only real ways to lose power, which is economic and political problems.
And large Empires cause both, due to the finances of patrolling and sustaining such a large military/organisation and often the political trouble of the conquered people resenting the imposed rules and rebelling. Something which costs more money, diverts resources etc.