What is the main conflict in The Eagle of the Ninth by Rosemary Sutcliff?

Marcus, a young centurion, begins his service in the Roman army of the second century with dreams of glorious military success and a triumphant return to the Etruscan farm that his family has owned for generations. Instead, he soon confronts feelings of alienation when his mother marries again and he is assigned to far-off Britain. Isolated from his family and country, Marcus must establish his own place and accept the realities of a sometimes harsh world.