Soldier's Heart by Gary Paulsen, is in the genre of a historical war novella. The novella follows Charley Goddard through three main phases of his life.
First, Charlie makes the decision to join the Union Army at the age of fifteen. He believes that joining the war effort will make him a man. As a man, he believes that he should be able to take care of his mother and the money he makes from joining will be money he can send home.
After Charlie gets to Fort Snelling he realizes that he may not have what it takes to stick out his decision to join the Union Army. The conditions, for Charlie, are deplorable. The clothing, food, and weaponry are far from even being sub-standard. During his enlistment, Charlie matures a lot. He sees the truths behind war- the deaths, loss of life, and the fear of dying himself.
After the war is over, Charlie (like many young soldiers) believes that he has experienced and seen too much for his young life. Even though the war is over, Charlie is traumatized by what he saw and experienced. Charlie will never be the same.
Basically, the theme of the novel is a boy's coming of age. The movement of the novel speaks to the generations of boys who have seen the outcome of war and, at the same time, gives current generations a look at what life is like on the lines.