Which Marine Corps commandant is most associated with establishing the modern structure and mission of the Marine Corps?

The correct answer highlights General John A. Lejeune, who is widely recognized for his significant contributions to the modern structure and mission of the Marine Corps. Serving as the 13th Commandant from 1920 to 1929, Lejeune implemented reforms that shaped the Marine Corps into a more efficient and capable force. He is credited with developing the concept of "combined arms," integrating various military capabilities to enhance operational effectiveness.

Lejeune also played a pivotal role in establishing the foundational doctrines that would guide Marine operations, focusing on amphibious warfare, which became a hallmark of Marine Corps strategy during World War II. His influence extended beyond tactics to include a strong emphasis on the importance of leadership and the Marine Corps' unique culture and values.

In contrast, while the other figures mentioned had notable roles in USMC history, they did not have the same far-reaching impact on the establishment of the modern Marine Corps structure and mission as Lejeune did. General Thomas Holcomb, for instance, was Commandant during World War II but is not as directly linked to foundational changes. General William Halsey, a prominent Navy admiral, focused primarily on naval operations rather than the Marine Corps’ structure. General Robert B. Neller served