History has shown that empires crumble when they overextend. Think of the later Roman Empire when it had to suffer the costs, military and administrative, of patrolling and defending a difficult border that expanded outwards. The British Empire and those of interwar Japan and post-WW II Soviet Union are other examples.
Overextension of empire in the past was about holding physical territory beyond the borders of the nation-state. But, today, the "American" empire is economic: It must defend globalization of "corporatocracy": The global capitalist system which, in the past, has included the subversion of governments too many to mention that threatened it (for starters, the overthrow of Allende in Chile and Arbenz in Guatemala; and the plane crashes of Torrejos in Panama and Roldos in Ecuador).
But, the new global Empire--an "imperial postmodernity" (here, I am referencing Hardt and Negri in Empire)--has no borders (since it is global) and is decentered and deterritorializing, thus it encompasses a spatial totality that is found everywhere (and thus, perhaps, difficult to recognize because it is "nowhere").
My contribution to this is that, since (as Hardt and Negri state) the new Empire "creates the very world it inhabits" (how's this for subtle control?) and seeks "to rule directly over human nature"--it must do this by manipulation of the system of signs and symbols (semiology) through such endeavors as corporate advertising. The ads convince us that global corporations are all about extending the goodness of democracy and technology and services to everyone in the world. Notice it hides the social relations of production as if things get made without sweatshops and exploitation of workers in less developed countries.
In short, Empire is also a state of "global cognition" within the individual. We get convinced that "we" need it and its military. It's the Empire state of war-mindedness that says to us to buy more bombs and drones and military technology for "our" protection. Then, we can relax back into watching the corporate ads interspersed in the bread-and-circus of corporate entertainment.
The postmodern frontier that Empire patrols is within us.