Military Science Fiction is a potent and growing sub-genre of SF in general these days. Prominent authors of military SF are John Ringo, Eric Flint, David Drake, and David Weber. Military SF is just that: SF stories generally involving violent, bloody wars fought by grunt soldiers wearing immense spacesuits that would dwarf Darth Vader's and weilding blaster-cannons the size of bazookas against horrible, detestable alien enemies. This isn't Space Opera, a la Star Wars or Lensmen; think of Starship Troopers. The focus in military SF tends to be on how a military unit would function in a futuristic war, with futuristic weaponry; tactics are paramount and a badge of honor among authors of this type of SF is how clearly they can write a very complex battle. I actually don't read much military SF; I find that a little of it goes quite a long way. I did enjoy On Basilisk Station, the first of David Weber's series of novels detailing the adventures of Captain Honor Harrington (initials HH....not unlike another famous literary Captain, albeit a seafaring one), even though Weber's characterizations aren't very complex and he demonstrated a strange tendency to break off in the middle of a fairly tense action-filled sequence to provide a lengthy infodump of some sort. Military SF is related to Space Opera, but it doesn't tend to have the galactic scope, the grand sense of wonder, and the Eternal-Battle-of-Good-Versus-Evil thing that attracts me so strongly to Space Opera. But Military SF is very popular these days.
The book I finished yesterday, The Forever War by Joe Haldeman, can be considered military SF, but having been initially published in 1975 (with portions appearing three years prior to that in Analog), it is quite different from the military SF of today. Haldeman's book is told first-person by William Mandella, who at the outset is a Private in a military strike force that is training for a war that has just begun against the Taurans, a race of beings encountered in a system a number of lightyears from Earth. The enemy is extremely distant, and some might not even consider them much of a threat, but the war goes on -- and on and on, for years and years. Also complicating things is the nature of space travel itself: transit between star systems is accomplished via "collapsar jumps", which accelerate the ships to something like .9c (nine-tenths of the speed of light), meaning that the soldiers spend years moving from one battle to the next -- while, in keeping with relativity, the time progressing from their perspective is measurable only in weeks. The effect is that when the soldiers finally do get their chance to return home, they are returning to a home that has changed drastically from the one they left, a home which doesn't welcome them with open arms (or even welcome them at all). The Forever War, then, becomes a SF-based allegory on the American experience in Viet Nam.
Private Mandella gets to know his comrades only by very broad strokes, and many of them are killed horribly and in ways that are surprising for their timing and mundane nature. He rarely has any idea (and thus, neither do we) of the military strategy at work behind the actions of the troops, and he at times suspects that there is no strategy per se, other than "Lose no more territory than has already been lost". Haldeman also employs his science very well: in addition to time marching on for Earth while grinding to a near halt for the soldiers, he makes us realize that this same fact of spacetime means that the enemy and the humans may at times be fighting with one side or the other at a clear advantage. In this way Haldeman captures the uncertainty that made the Viet Nam War so harrowing for the soldiers who fought it, that feeling that one not only knows when the enemy may attack but where he may attack from. During the military scenes there is a constant sense that the Taurans may arrive at any second, and they often do.
Equally effective are the scenes on Earth, when Mandella returns home after his service is up. He discovers what seems to him a world gone mad. The Earth that Haldeman shows us is rather dystopic, but what is interesting isn't so much the Earth as Haldeman shows it as Mandella's response to that Earth, as he attempts to carve out a niche where he can live his life and, maybe, enjoy love.
The Forever War suffers a bit in its third act, which felt a bit perfunctory. Mandella is re-drafted into service, this time as a commanding officer. I think that Haldeman is trying to show that the war was just as choatic and horrible for commanders as it was for the "grunts". A bit of this comes through, and the trials-and-tribulations that Mandella experiences in his first command post -- for which he really isn't cut out -- are interesting, but this part of the book still seems to be striving too hard toward the massive Final Battle, which is almost obligatory in stories like this. Haldeman's ending, too, seems a bit too easy and a bit too happy after such a harrowing book. But it does work, though, being based as it is squarely on the science that forms the basis of Haldeman's entire enterprise. Even though I'm not sure if things should work out the way they do, I'm glad that they do.
The Forever War is a very impressive work.