I imagine that if a nation-state really wanted to get serious about space warfare, they would have to create an entirely new branch to do it. The situation up there is just too different from anything they've played with before.
I thought that a while back, however I've changed my mind. I assumed Nasa would become a Military branch or something else would take that place, like you said.
Here's why I changed my mind:(other then it actually being in the court of the Air Force, like I said before) When we think of the Military (well, I actually mean me but I assume everyone else in the world will automaticly agree with my unquestionably superior point of view) *cough* anyway, when WE think of the Military, WE think of the Air Force bombing, the Army or Marine Corp. slugging it out on the ground, and the Navy trading shots with enemy ships.
However, that's closer to a Battlefield (as in the EA game) situation then reality. I've noticed that every branch is well-equiped to take on a rival nation by itself, even though they do work together a lot. (The Marine Corp. having to steal medics from the Navy for instance) The Navy, oxymoron as it is, could fight across the land. They have their own Air Support, their own ground troops (Seals being only one example). The Air Force, more recently I think, is fighting on the ground more and more. They have their own grunts for defending bases, police, and such just like the Army. They may not have them in as great supply as the Army (I really have no idea how many of them there are) but they could easily do pretty much the same thing. The Marine Corp., as seems all the rage these days, has it's own Air support, and, well, it's the freaking Marine Corp. On top of that, they all have their own spec ops crews meant to do anything- Don't think Seals were just the Navy's idea. Any branch of service could put men on the field trained to fill in and do similar things. Think of The Red/Black Barrets, Delta Force, Army Rangers, Pararescue, Force Recon, to name the few I can remember)