America has the foreign policy and military structure of an imperial power, but lacks a direct mechanism to collect the tribute that historically supported empires. Instead, the system depends on the US possessing relative economic advantage in a free-trade system to create prosperity which can then be taxed, underwriting the enormous military budget. That system has been broken since the late 60s; globalization and radical antitax conservatism eliminated the financial foundation required to maintain such a vast military. In the end, a state that can't pay for its military ceases to have the ability to act as a hegemon.