Which document established the foundational structure of the Marine Corps?

The National Security Act of 1947 is the document that established the foundational structure of the Marine Corps. This act restructured the organization and responsibilities of the U.S. military, formalizing the role of the Marine Corps within the Department of Defense. It notably created the Department of the Air Force and unified the military under the National Military Establishment, which later became the Department of Defense. This legislation solidified the Marine Corps' status as a distinct and essential branch of the U.S. armed forces, ensuring its integration and coordination within broader national defense strategies.

In contrast, while the Constitution, foundational to American governance, outlines the establishment of the military in a general sense, it does not specifically define the structure of the Marine Corps. The Maritime Strategy Document, focusing on naval and maritime operations, does not fundamentally establish the structure of the Marine Corps itself. Similarly, the Unified Command Plan pertains to the organization of combatant commands rather than the establishment of agency structure, which makes it less relevant in this context.