What department is responsible for national defense?

The Department of Defense is responsible for national defense, which is its primary mission. This department oversees the armed forces and is in charge of coordinating and supervising all agencies and functions of the government relating directly to national security and the military. The Department of Defense’s role includes preparing for and managing military operations, ensuring national security policy is implemented, and protecting the country's interests both domestically and internationally.

In contrast, the Department of State is primarily focused on foreign policy and diplomatic relations, the Department of Justice deals with law enforcement and legal affairs, and the Department of Homeland Security concentrates on protecting the United States from domestic threats and ensuring secure borders. Each of these departments has its own specific functions, but only the Department of Defense is directly tasked with the comprehensive responsibility for the national defense of the country.