I think a lot of Americans might be surprised to hear just how constrained the British forces will become. Despite British claims, the fact is that right now the United States has virtually the only expeditionary force left on the planet. Other countries can defend themselves, or send troops across a border, but there's almost no capacity left for projecting force any further than that anywhere in the world. This is one area where American exceptionalism is truly a fact.
What this does to any idea of collective intervention or intervention with even a veneer of international legitimacy is surely profound. And that in turn exposes and isolates the US hegemon even further. It will make every argument for a new war an argument solely about the projection of American force, and make the endeavors seem even more neo-imperial. If you combine this with what will have to be real cuts in defense if the US is to avoid default, and the unavoidable disasters of the Iraq and Afghan adventures, then the American century, in terms of global control and global legitimacy, really is receding faster than many now realize.