Chalmers Johnson, one of the key articulators of the "imperial overstretch" argument (I liked his book The Sorrows of Empire: Militarism, Secrecy, and the End of the Republic) argues that military Keynesianism and an unchecked presidency on the warpath will lead to a non-democratic United States and, eventually, bankruptcy. From Harpers:
The United States remains, for the moment, the most powerful nation in history, but it faces a violent contradiction between its long republican tradition and its more recent imperial ambitions.
The fate of previous democratic empires suggests that such a conflict is unsustainable and will be resolved in one of two ways. Rome attempted to keep its empire and lost its democracy. Britain chose to remain democratic and in the process let go its empire. Intentionally or not, the people of the United States already are well embarked upon the course of non-democratic empire.
Several factors, however, indicate that this course will be a brief one, which most likely will end in economic and political collapse.