What is the main conflict in The Rise of Theodore Roosevelt by Edmund Morris?

The Rise of Theodore Roosevelt skillfully catalogues the early years of the man who eventually becomes president.

Born in New York City to Mittie and Theodore Roosevelt, their second child and first son suffers from delicate health immediately. Theodore Senior responds to this with admonishment that his son pursue as vigorous a lifestyle as possible. Roosevelt rises to the challenge and, against doctors orders, strengthens his body with a variety of physical pursuits.