The Forever War

Battle suits, space travel, intergalactic war between humans and the Tauran race. Very space opera with a quite liberal view on sexuality mixed into it (plus a not-so-healthy dose of homophobia). What does it mean to return to earth after 20, after 100, after 300 years when speed-of-light travel kept you young while everyone else aged and culture underwent radical changes. The story has our protagonist rising through the ranks, we observe him in several battles, detailing battle and survival strategies, dealing with dissent among troops, having to make decisions that likely cause casualties. Decent. Not mind-bending.