Joe Haldeman 
Forever War #2 [PDF ebook] 

As they touch down on the planet Aleph, Platoon 1 are about to put everything they’ve learned in training into practice. They are under orders to engage the enemy — the Taurans — whom none of them have ever seen before. Ready for battle, they set out across the alien world. The war is real now. They’re in it.But, like everything, the expectation and the reality are markedly different. The ideology that led them to this situation also led to decisions that none of them could have prepared themselves for — and the lucky ones who survive will never be the same again…