The US government’s obligation to its own people is mutually exclusive to acting on behalf of the international community.  A government derives its sovereignty from a social contract with its citizens. Citizens surrender some of their freedoms in exchange for government protection; if a government does not serve its people’s best interests, it is not legitimate. Thus in any situation where the interests of the American public are not aligned with those of the global population, the US military cannot serve the international community without failing to meet its obligation to its own citizenry. Because the American public has the ability to oust a leader that does not promote their interests, the military is much more likely to choose the option of serving American interests. This may not be unreasonable behavior, but it is indicative of the need for other entities- either other nations or international organizations- to have comparable military power to that of the United States.