This links to a PDF of slides from a briefing by Dennis M. Bushnell, Chief Scientist at NASA's Langley Research Center, based on work done for an alphabet soup of military and intelligence organizations.
Basically, due to simultaneous revolutions in IT, biology and nanotechnology, it's going to be increasingly more difficult to protect the US. Technology and the spread of information will empower our competitors, our enemies, and even individuals. This is no surprise to longtime Astroroach readers, but Bushnell's presentation really slams the point home. The world is changing because knowledge is ever more available to anyone who desires it, and knowledge is more than power - it's everything. The playing field is leveling.
As a result, the presentation doesn't focus on who our adversaries might be, but what their capabilities will be. It also suggests how these might be countered, but I have to say that it doesn't look promising. For example, one of the suggestions is to "work on and advertise a really effective retribution (a la MAD) to deter delivery", but I'm not convinced that fear of retribution will stop religious zealots out to bring on Apocalypse. Preemptive actions could help, but as we've seen this has it's own set of problems.
The last slide lists "Usual Reactions To This Presentation" which range from disbelief to "let's hope they don't do that" to the attitude that we can't defend against it so why try - it's too hard. Yep. We're screwed.