How do I formally address an officer in a letter?

To formally address an officer in a letter, begin with the appropriate salutation based on their rank. For example, use "Dear Officer [Last Name]" for general officers, or "Dear Captain [Last Name]" for captains. If you are unsure of their rank, "Dear [Rank] [Last Name]" is a safe choice. Always ensure to include a formal closing, such as "Sincerely" or "Respectfully," followed by your name.