Conversation so far:
Was Lord Voldemort in the film Harry Potter and the Half-Blood Prince?
Lord Voldemort does not appear in Harry Potter and the Half-Blood Prince.
How was Lord Voldemort's presence felt in Harry Potter and the Half-Blood Prince?
Lord Voldemort presence and actions are felt when he once again declares war, and begins to rise to power once more.
What happens as Lord Voldemort gains power in Harry Potter and the Half-Blood Prince?
Lord Voldemort murders Amelia Bones of the Department of Magical Law Enforcement, and begins to target members of the Order of the Phoenix, including Emmeline Vance.
What happens next in Harry Potter and the Half-Blood Prince after Voldemort murders Amelia Bones and targets members of the Order of the Phoenix?
J. K. Rowling uses several chapters in Harry Potter and the Half-Blood Prince as exposition to establish Voldemort's backstory.

Question: How does she do this?
Self-contained rewrite: How does J. K. Rowling establish Lord Voldemort's backstory in Harry Potter and the Half-Blood Prince?