Last year I wrote posts on, respectively, Robert A. Heinlein's Starship Troopers and Joe Haldeman's The Forever War.
More recently, I've read John Scalzi's 2005 novel, Old Man's War, which can be understood as an updated version of both. I haven't read any of the book's increasingly numerous sequels, so I'm going to comment on it isolation.
Old Man's War has received much attention over the past decade or so, and it's widely regarded as one of the most important science fiction novels, so far, of the new century. On balance, I think that response is justified, and that the book is in many ways a worthy successor to Starship Troopers and The Forever War. To be honest, I found much of the dialogue and exposition too hokey, but the story is involving and suspenseful. Some irritations aside, it held my attention.
Like the novels by Heinlein and Haldeman, Old Man's War is told in the first person by a military recruit - John Perry in this case - telling us about his enlistment, training, combat experience, and other (mis)adventures. As with the earlier novels, there is much emphasis on the daily hardships of military life, though it is clear enough that the narrator has at least survived to tell the tale.
Scalzi updates the vision of futuristic military technology: instead of emphasizing armoured battlesuits, he portrays his warriors as radically enhanced via genetics and other biological innovations. Although they do wear high-technology suits as protection, they rely mainly on superhuman strength, speed, reaction times, and so on, together with wonderfully advanced guns and communications systems.
The major twist is that the military of the future enlists older recruits: enlistment is for volunteers who have turned 75. On joining up, they are augmented by having their minds uploaded into biologically enhanced clones of themselves, physically aged about 20. Though the exact nature of what will happen is not revealed to them in advance, they do receive strong hints that they will be rejuvenated in some way, and indeed this is their major inducement to enlist. The rejuvenation is simply much more radical than they expect or can imagine. (In this novel at least, Scalzi does not deal with the issue of whether the recruits' identities are preserved by the uploading process.)
As with Starship Troopers, there is no end in sight to the war, although in this case it is fought against a variety of alien civilizations squabbling with each other, and with colonists from Earth, over territory - or in the case of one especially highly advanced civilization, fighting for less scrutable reasons. At the same time, we are left with hope for an optimistic outcome for the main character (similar to that ultimately gained by William Mandella at the end of The Forever War).
In writing, last May, about The Forever War, I noted how its portrayal of the military - and of warfare - is far more bitter and cynical than that of Starship Troopers (which could surely be used as recruitment propaganda). Yet there is a sense in which Haldeman's book has a happier ending than Heinlein's, since its war is not actually "forever". Old Man's War neither glorifies the military and a commitment to military life, in the way of Starship Troopers, nor condemns them, in the way of The Forever War. If anything, Scalzi makes the difficulties and the tedium seem less oppressive than we are shown by either Heinlein or Haldeman - though there are plenty of deaths of sympathetic characters, and the narrator is put through numerous kinds of hell. The difference is not so much in the kinds of events described as in the distinctive voice of John Perry: he seems detached and intelligent both in his role as a narrator and during the various episodes that he recalls (this is, perhaps, appropriate for a man of more-than-mature years who has had time to acquire much worldly wisdom before he even signs up).
As Old Man's War ends, war seems to be neither a necessity that transcends any moral demands (providing, rather, a backdrop to them), as in Starship Troopers, nor to be, as in The Forever War, sheer unnecessary craziness. The sense, rather, is that pursuing alternatives is difficult, perhaps almost impossible. We need to try, but meanwhile we also need to carry on. One character who preaches diplomacy rather than war is shown as both essentially right and as a sanctimonious, egotistical fool (there may be a way to achieve what he wants, but not here or yet or easily... and he's a fool not to realise that).
That's a more complex vision than we're used to in science fiction novels, perhaps, though possibly a realistic one; I'll be interested to find out where Scalzi takes it in the sequels.