You can say pretty much everything worth saying about powered armor space marines between The Forever War and Starship Troopers. Joe Haldeman's The Forever War covers the same ground of training, combat, recuperation, and command as Heinlein's novel, but from a pacifistic and detached post-Vietnam perspective. William Mandella, our hero, is a reluctant soldier, an "elite draftee" with an IQ over 150 and a physics background sent out to fight an unknown alien enemy, the Taurans, who have been hitting human colony ships. FTL involves jumps through collapsars, black holes with orbiting planetoids, so Mandella and his comrades are trained to fight in extreme conditions just above absolute zero, with seas of liquid helium and deadly hydrogen ice sheets. The plan is simple: land on a planetoid, kill any Taurens, construct a bunker and laser installation and hold till relieved.

Of course, the first rule of all military activity is SNAFU, and for their first mission, Mandella is sent to a jungle world at near boiling temperatures. Their landing site is mile underwater (fortunately their dropships are submersible), the local wildlife is telepathic, and after an unprovoked attack kills the platoon's telepathic sensitive and spookily shadows them. The Tauren's don't fight back, but one escapes in a personal spaceship. Despite the lack of resistance, some of the squad is killed by anti-air weapon. Mandella is disgusted by the use of post-hypnotic suggestion to make him fight. A second encounter in space goes poorly for their cruiser and they retreat, with one of my favorite lines in the book "...surely the Captain was not possessed by something so unmilitary as the will to live." This is where one of the central conceits of the book is introduced. Though FTL exists, relativistic maneuvering around collapsars and fighting in the warped spacetime on portal planets dilates time for soldiers. Soldiers, even if they survive, can never really go back to a planet that has experienced decades of time to their subjective year-long tour. Worse, enemy forces can come from your subjective future, with the benefits of extra R&D. Technically, this advantage applies to both sides at random, but that's cold comfort when the enemy shows up with a superweapon you've never seen and have no counter for.

The second chunk of the book was stripped from the original version (I'm reading the 1991 complete edition), and follows Mandella on an Earth that has gone downhill since he left. His mother is 80 years old, a food war killed billions, and the survivors are equally victimized by a powerful one-world government which controls food, power, and jobs, and criminal factions which provide necessary work-arounds to the system and random criminal violence. Mandella and his lover, lost on Earth, re-enlist on promise of a safe training job and are immediately reassigned to combat. Mandella is no hero, but a knack for survival gets him promoted to Major and strike force command. By now, he's separated by centuries from the troops, who are creche raised and all gay, with heterosexuality treated as a curable deviance. Command is no picnic, Mandella is profoundly alone and untrusted by his troops, and separated forever from his lover. He sets up a base on a larger than average portal planet in the Magellanic Cloud, survives one last battle, which features a lone fighter making an attack run at .999c that destroys the enemy cruiser and shatters Mandella's bunker with an earthquake, and returns home to find that the war is over. Humanity has been replaced by Man, a race of clones, which has reached a peace settlement with the Taurans, also a race of clones. The whole war was a lie, the initial attack faked by UN high command who thought a war was just what Earth needed to kick it out of an economic depression. Baseline humans have settled space, and Mandella's lover Marygay, has also survived, using an obsolete cruiser as a relativistic shuttle until he returns.

Some closing thoughts: Haldeman is obviously a talent. He wrote this book in his late 20s (serialized in 1972, novel in 1974) as an MFA thesis at the Iowa Writer's Workshop, which is the major influence on post-war American literary fiction. It's a personal novel as well; Haldeman is a Vietnam veteran with a physics degree, his wife shares a name with Mandella's partner. While Starship Troopers takes war as necessary to glorious, The Forever War sees it as dehumanizing and full of lies. The basic incompetence of commanders, and the numerous ways in which they screw with ordinary soldiers, is a repeated theme. The mutual alienation of soldiers, the society they are "defending", and the reasons for the war, are all directly translated from the Vietnam War. The social side is also fascinating. Mandella's army is a grunt's fantasy, 50-50 coed with willing combat females, legal marijuana after hours, and "Fuck you, sir!" as the mandatory closing refrain. Many changes on Earth are only sketched at, but the shift to mandatory homosexuality as a birth control measure is handled pretty well for a novel written back when being gay was still technically a mental illness ("...you think you're tolerant, sir.") But one of the coolest scifi points, and one which is easy to overlook, is the way The Forever War plays with temporality. While the Vietnam War as a whole seemed to go on forever, individual soldiers were acutely aware of how much time they had left on their 365 day tour, unlike the space marines who are unlikely to ever see the end of their two-year subjective enlistment. The subjectivity of time is another interesting point. In Vietnam, everybody's tours counted down the same, whether you were safe running a PX in Da Nang or an airmobile machinegunner who might see 300 days of combat. Time's the thing.