One possible answer is "relative to what is necessary for the system to sustain and reproduce itself." Many nations and civilizations were brought down not by its outside enemies, but by their inability to secure resources (including human and political capital) that are needed to sustain their existence the level that defines them as unique cultural entities.
Take for example Rome whose military, political, economic and organizational capacities stood head and shoulder above those of the surrounding "barbarians" - yet it was internal decline that made Rome more susceptible to challenges from inferior enemies - challenges that Rome could effectively meet when it was internally strong. More extreme examples as the Mayas and the cherished Easter Island, which collapsed under their own weight with little or no help from external enemies.
Just think what will happen when the empire will no longer be able to deliver expensive goodies and life styles that makes the US populace content - vast stretches of suburban wasteland, MacMansions, SUV's powered by cheap gas - at least at a price that the populace can afford - at the same time demanding more sacrifices from the very same populace - stagnant or declining wages and living standards, lower level of services, more draconian laws, higher demand for cannon fodder, etc. How long do you think the empire will be able to maintain its internal cohesion without various groups being at each other throat's (they already are!). Don't you think that this will create ample opportunity even for much weaker enemies? How long do you think the Chinese and the Europeans will be willing to finance borrowing that maintains the vastly wasteful US economy and its expensive military machine?
Wojtek