The Forever War feels a lot like Halo, with Captain Mandella playing a novice Master Chief battling against the Covenant looking Prophet less Taurians. For a novel written in 1976 on horrors of Vietnam war with an Interstellar undertone, the hard science feels so surprisingly contemporary and expedient. Or accurately prognostic. 
 Sci fi is often metaphorical in nature, and in this novel Haldeman accounts the horrors he experienced in Vietnam war, without recreating it essentially. I don't think any other linear work directly dealing with the subject of war could have accurately conveyed the essence and remained ageless. Another book I know which did a same job, though not similar is Slaugterhouse Five, which depicted war through a non nonchalant non linear life, where everything felt preordained with time travel. 
 Humanity is at war with Taurians, an alien civilization we know very little about except the fact that they initiated the conflict on very first contact, attacking Earth's outpost in farther space. Planet's elites, the ones with IQ above 150, are soon absorbed by UNEF and then send to Charon for revenge and recon. Many couldn't survive the rigorous training itself, and a lot more were killed during the first post-hypnotically suggested initial combat, thanks to the alien environments and weaponry. Soldiers travel intergalactically through Collapsor jumps (worm holes), with enigmatic relativistic effects, by the order of centuries back on Earth, making them alien to the world whose very future they've been fighting for. 
 Couldn't help but notice the areas Interstellar borrowed from this one, including the final letter between Marygay and Mandella. Even though this was a dystopian military sci fi novel, it had a pleasant aura around it, and subtle humour, with a very satisfactory ending. Like Peter.F.Hamilton says on the cover- "damn near perfect."