War is hell, and it fundamentally changes all things. When you are asked to fight in a war, the horrors you see inevitably change you. When you are entrenched in a long bloody war abroad, the world you leave behind moves on without you. If you are lucky enough to return with your life, you find yourself a stranger in a familiar land. This is the reality of all wars.
Now consider what would happen if you are fighting an alien enemy light years away from your world. Imagine there is no FTL space travel and you can only travel just under the speed of light to reach the battlefield. Each time you engage the enemy, the month long excursion results in hundreds of years passing back home. What do you do when every time you return home from war everyone you have know is dead and gone? What do you do when the world you return to is almost as alien as the enemy world you are fighting against?
If you read The Forever War you will see an excellent glimpse at what that reality looks like.
WARNING SPOILERS AHOY
The disorienting effect of time-dilation plays a key plot-device in this story. Our hero, William Mandella, only fights in about four battles during the entire war. But those battles take place over the period of a millenium back home on Earth. By the end of the war Mandella has become a legendary solider from Earth's ancient past. The Earth he returns to owes him a massive debt for his service, but also can't help looking at him as backwards and an esoteric dinosaur of their society. But imagine how he feels?
Soliders who fight in foreign wars know this feeling all to well. This is one of the reasons I love Science Fiction so much. SciFi lets you exaggerate an important social issue and then highlight the ramifications of its effect on our daily life. It's not just about spaceships and laser guns. It's about looking at important social issues and using speculative science and technology to build a world that establishes the challenges humanity always faces.
War changes things. Sometimes it changes the solider. Sometimes it changes the world you are fighting for. In The Forever War we get to see the world change so much that the victory almost feels hollow against everything sacrificed in order to win. But at the end, you have to ask if this is how the veterans of the Vietnam or Iraq war felt after coming home from several long tours. I'm not a solider, so I'll never be able to answer that question. But I love that this book helped me consider these things, while being a fantastically told story with rich characters I can connect with.
If you haven't read The Forever War and you love SciFi, I implore you to read the book.