Conscripted into a war in which you have no clue as to why you are there. This was the feeling of the soldiers of the Vietnam War, excuse me, Police Action. The government of both societies fired before asking questions. Then they put people in front of the firing squads of the other sides. A government that does not see these people as people, but rather as numbers, as another mindless weapon. The government of Forever War was not above putting hypnotic suggestions into the minds of the troops to make them fight.
The men and women were put through a rigorous training that seemed to not only include training in weapons but also in dealing with death. On the front lines, very few soldiers were expected to come back. Those that did made their way, sometimes unwillingly, through the ranks. Bad military logic (can there be any good military logic?) said that since they were able to survive through an attack, they must have some skills that would allow them to lead others. This is how Pvt. Mandella became Lt. Mandella and even Major Mandella. He was one of two to survive the entire war, not because he was any good at leading or fighting, but because he was lucky.
Even his last battle, which was fought after the official end of the war, he stumbled upon a solution that would save some of his men. This also has military precedent. The slow communications that existed because of the method of traveling through space made it difficult for messages to get through to the troops. They never even know what the technological level of the enemy is going to be, Mandella even thought that "the Taurans could say a word, and his battalion would disappear." In the early 19th century, communication was also very hard. The most decisive battle of the War of 1812, the Battle of New Orleans, occurred months after the end of the war officially happened.
This book was a protest against war by Joe Haldeman, a war veteran. He was in Vietnam, and knew how the soldier felt. He saw what this might become if the government continued on its policies regarding staffing the wars.