atm, the American position is strategically unassailable. only the US has a navy sufficient to project power to any of the world's oceans ... and win the conflict there, no matter how far it is from American shores.
in the long run, that, of course, is not true. America could give away that position via any number of means.
It seems unlikely that any other country will go to the trouble and expense of building a navy that powerful -- which suggests that the route to such power in the future is likely to be something more sci-fi ... perhaps hypersonic space to ground missiles in permanent orbit 23,000 miles up. They'd be capable of delivering warheads anywhere in under 10 minutes.
similarly, ground launched hypersonic weapons could probably be built in twenty years or so which would be capable of striking anywhere in 20 minutes. Obviously, only a nation that invests heavily in rocket science could possibly do this and Russia seems to be the only candidate with a program sufficiently active to accomplish the task. [assuming, of course, the the US does not have a black project already underway to do exactly that.]
this raises the possibility that the next war will be one of hypersonic strike vehicles.
there is still the chance that some smaller nation will produce an EMP device and be able to orbit it -- detonating directly overhead of America. This would knock the American civilian economy back to the 1870s or so. We can only hope that our government is aware of the possibility and taking steps to prevent its success. [they aren't telling us.]