What is the ultimate authority beneath the President for national defense?

The ultimate authority beneath the President for national defense is the Department of Defense. This department is responsible for coordinating and supervising all agencies and functions of the government related directly to national security and the military. The Department of Defense encompasses all branches of the armed forces, including the Army, Navy, Air Force, and Marine Corps, which are vital in executing the President's defense strategy and policies.

The organization plays a crucial role in ensuring readiness, managing defense operations, and formulating defense policy. It operates under the direction of the Secretary of Defense, who is appointed by the President and serves as the principal defense policy advisor. This chain of command highlights the significant role the Department of Defense plays in supporting the President's authority in matters of national defense.

Other options like the Department of the Navy, Homeland Security, and the National Security Agency have specific roles within the broader scope of national security. However, none coordinate defense efforts across all military branches and agencies as comprehensively as the Department of Defense.