Because the GPL (and other such) is a license, that can apparently have almost arbitrary restrictions, can there exist a NMPL -- non-military public license -- that excludes military use? For a current and real example, much commercial software is licensed under terms that exclude use in nuclear or lifesaving critical applications.

I'm not sure how one enforces such a license. Let's be real, even a successful lawsuit isn't going to remove said software from the military's hands. And to the extent it is honored, it has the negative effect of driving up the cost of developing military software, which of course comes out normal people's pockets.

But it might be an interesting way to take up a principled stance.