Wood, EM. 2003. Empire of Capital. Verso.
If one makes arguments of the nature that the US is an imperial body, they are routinely countered that the US does not control any territory. This is, of course, true. However, this misses the fact that there is an imperial force at work in the world, that it originates and derives power from the system hegemon and that it all operates within the capitalist system.
Wood highlights the driving power of capital to continually search for higher returns as being the ultimate source of global imperialism. This imperialism does not have the same characteristics as did previous imperialistic eras; no longer is there a search for territory or direct rule. Instead, this imperialism is a control of a different and more amorphous type: the logic of capitalism.
Military expansionism is another piece of this puzzle: the US in particular possesses massive amounts of military power, and this is used as the ultimate prop for regulatory frameworks that are necessary for the further promotion of capitalism.