Buffalo Soldier by Tanya Landman

When Charley is freed from slavery at the end of the Civil War between the Yankees and the Confederates she imagines a new world of unlimited opportunities. Instead, she finds a life that is more dangerous than ever before. Her only way to survive is to disguise herself as a boy and join the army. But the army, like everywhere else, is riddled with prejudice and danger. It is only when Charley is sent to fight against the Apache Indians, another much discriminated against group, that she begins to learn what is could mean to be free.