What significant change to the Marine Corps occurred in 1947?

In 1947, the establishment of the National Security Act marked a significant change for the Marine Corps by formally establishing the Department of Defense and streamlining the command structure of the U.S. military, including the Marine Corps. This act not only elevated the status of the Marine Corps but also provided a framework for its role in national defense, integrating it more closely with the other branches of the armed services. The act codified the functions of the Marine Corps and solidified its position as a key element of U.S. military policy and strategy in the post-World War II era.

While the other choices represent important historical developments within the Marine Corps, the National Security Act was pivotal in shaping how the Marine Corps functioned within the broader context of U.S. military operations, emphasizing its role in unified defense planning.