What formal document established the Marine Corps Doctrine?

The correct choice is the National Security Act of 1947, which formally established the framework for the United States' military forces, including the Marine Corps. This act restructured the military, creating the Department of Defense and providing a clear doctrine that guides military operations and organization. It helped define the role of the Marine Corps within the broader context of national defense policy.

This act is significant because it lay the groundwork for how the Marine Corps and other branches interpret their functions, roles, and responsibilities in alignment with national security objectives. The incorporation of the Marine Corps into the National Military Establishment shifted its operational doctrine, ensuring that it was cohesive with the overall strategy of U.S. defense.

The other documents mentioned do not serve the same purpose. For instance, the Marine Corps Manual provides guidance and procedures for the operational capabilities of the Marine Corps but does not establish doctrine at the level the National Security Act does. The Constitution of the United States outlines the fundamental principles and framework of government but does not detail military doctrine specifically. The Uniform Code of Military Justice (UCMJ) governs military conduct and legal matters but is not a source of doctrine for military operations or organizational structure.