Earlier today, I had a post go up onto io9 titled 'What Is Military Science Fiction?', with the intent of looking at what defines books that are generally classified as such. The end result? While there's a lot of books that look towards the military for their stories exclusively (Think Baen Books), there's a second grouping of books that utilizes the military to an extent, some of which are somewhat surprising, like Dune and The Moon is a Harsh Mistress, which generally aren't thought of as such.
The general problem that I see with military science fiction is that they're not about the military: it's about the people in the military, and their (or society's) experiences with warfare. This seems to be a large gap in understanding, given how much warfare generally impacts fighting in American lives: if you go back through U.S. history over the past century, you would have to go to the 1980s before you found a year that the country wasn't engaged in military operations, publicly. Including classified and forgotten operations, you'd be hard pressed to find a time when we haven't been shooting at someone. Thus, it's essential for an understanding of why people go to war, especially in the speculative fields, because it's such a major impact.
One of the more enlightening quotes that I pulled comes from William Gibson: "Every fictive, imagined future can only be understood historically within the moment it was written." This is huge, and it explains everything that art tries to do in every field: it explains the present by looking at how we comprehend our surroundings. Looking at a lot of the military SF field, it's no wonder that the Second World War plays such an impact in the speculative military fields; it plays a huge role in our lives now.
Beyond that, however, it's no wonder that we look to futuristic warfare to understand the present. Science Fiction helps to take the present out of context, so the reader understands things from another perspective. Joe Halderman's The Forever War looks at experiences that are closely linked to coming home from Vietnam, with a sort of culture shock from returning soldiers.
Other authors have realized, through other works, the roles that politics and radicalism play in the greater picture, and this is why, I think, some books aren't thought of as being military-centric: the actions that surround warfare, such as the political motivations and societal issues, aren't directly linked to war and fighting, but help to give it some context, something that is badly needed in a lot of stories.
The ongoing war in Afghanistan and Iraq are two wars that require a lot of context. In Iraq's case, one has to look closely to American foreign policy and political circles to best understand the motivations for going to war (agree with them or not, there are some deep seeded reasons, not all of them bad, for going in). There is likewise a necessary examination required to look at Osama Bin Laden's motivations for forming Al Qaeda, which in turn links to other things. War is not a clear-cut and dried action or series of actions, and unfortunately, it is often presented as such.
This is why, I think, Military Science Fiction isn't all that great at being able to predict where warfare will go: there is already a lot of disagreement amongst historians and theorists as it is, over elements that lie in the background of the fighting, while speculative elements look towards the technology for inspiration, while they should instead be looking more at how people think about, and actually conduct warfare.
The ongoing battles going on right now across the world are way more science fictional than any sort of story that we've been able to come up with. Insurgents operate in ways that are vastly different than anticipated. A terrorist armed with a mobile phone, the internet and all the resources of the planet, guided by radical ideas fed to him by someone he's never met can go and become part of a fight and push forth a political message. If there's ever a scary, speculative story that keeps someone up at night, that should be it.