Camo & Stealth
It has long been said that nanotechnology will first be used by the military, just as all new tech seems to be used. Things such as cell phones and GPS began with military use first, as just a few examples of this. Nanotechnology has also been assumed as the answer to the development of better camouflage and stealth technology that can essentially make a person impossible to see. However, the same tech that gives us something like this can also create more powerful military weapons and defense systems. Sure, this will bring about massive advantages for militaries. However, that is also the problem in a nutshell. It will be used not just by the U.S. Armed Forces but also by places such as Russia and China. We could literally start a new arms race with this stuff.