The Forever War by Joe Haldeman has been called the best (or second-best, after Starship Troopers) sci-fi war novel ever written. That might be right--it's certainly obvious that Haldeman had been a solider himself.
Some critics call this book a direct Vietnam allegory. It certainly lends itself to comparisons: drafted soldiers, a war that goes on far longer than planned, travel far from home and a home that has drastically change when the soldiers finally return... leaving little room for them in it.
The details are a lot of fun. Their military is undramatically co-ed--not only do men and women serve together, but they are assigned shared bunks (men with women) and encouraged to enjoy sex within the ranks. The social ease and routine of this bonding initially casts the main relationship as less "romantic" than I've seen, and makes later social developments harder on Mandella and his fellow soldiers.
Mandella is not remotely a warrior by temperament, and the horror of war through his eyes is even more sickening than it might be otherwise. It's an example of the desperation of this war that they draft men like him not only to fight but later to strategize. He knows how boneheaded it all is, and eventually takes it with dry--if not good--humor.
The most painful part of the book is the middle, when Mandella's company is allowed to return to Earth after their stint in the war. Radical changes abound. This war has been fought by traveling through wormholes and by utilizing relativistic speeds. Mandella is ten years older, but everyone else is close to 40 years older. The war has forced drastic changes: rationing of food, apartments, jobs and health care; heavy police presence to counter the rise of black-market organized crime; a return to communal living in some areas; and a government-encouraged rise in same-sex coupling. (This last is supposedly for birth control; Haldeman's writing implies that government support has gone hand-in-hand with legitimate social change--by the time Mandella gets home, same-sex couples are seen as natural and romantic as straight ones.)
The state of Earth is too much for Mandella. He and his fellow-soldier-girlfriend, Marygay, decide that the world doesn't have enough room for them. What they do know how to do is make war, so... they re-enlist.
But running away from change, even back into war, provides no shelter. And when those changes infiltrate even war... love becomes the only basis for sanity.