Say you wanted to add a clause to the apache license (AL2) or GPLv3 - Then you would also have to make sure that your new clause does not contradict anything in the rest of the license.
Edit: And that clause would probably prevent even others who are not in the military from using your code. Just because there's a chance your license is incompatible with the GPL.
The longer the license, the more likely it is that you have not understood the effects.
The simplest license is abandonment to the public domain; that won't work. You need something that establishes that you are the original author (so that you own the rights) and that you are offering certain specific rights to most people (now it's open source, though not Open Source) and that keeps the same restrictions for people making derivative works -- and now you have the BSD 3-clause.
So that's the simplest license which can possibly work for the OP's purposes.