In which war did the Marines incur the highest number of total casualties from all causes?

The correct choice reflects the extensive involvement of the Marines in the conflict within Southeast Asia, specifically during the Vietnam War, which lasted from the late 1950s to 1975. This period was marked by a high level of engagement and intense combat, leading to significant casualties from combat and other associated causes.

While World War II, the Korean War, and the Gulf War also resulted in sizable Marine casualties, the duration and scale of the Vietnam War positioned it as a particularly costly conflict for the Marine Corps. The combination of prolonged combat operations, guerilla warfare tactics employed by the enemy, and difficult terrain contributed to a higher casualty rate during this conflict. The nature of the war, including combat-related injuries, illness, and the psychological impact of prolonged deployments, further exacerbated the casualty figures.

Thus, the context of the Vietnam War's complexities highlights why it stands out as the conflict with the highest total Marine casualties from all causes.