Which Marine served the longest term as Commandant?

The correct answer is Archibald Henderson, who served as the Commandant of the Marine Corps from 1820 to 1859, making him the longest-serving Commandant in the history of the Marine Corps. His nearly 39-year tenure is notable not only for its duration but also for the significant impact he had on the development and modernization of the Marine Corps during a critical period in American history. Under his leadership, the Marine Corps evolved in terms of size, organization, and operational capabilities, which set the foundation for its future roles in various conflicts.

In contrast, General Pate, General William Westmoreland, and Captain John Fitzpatrick did not serve nearly as long as Henderson. Westmoreland is primarily known for his role during the Vietnam War and was not a Commandant, while Pate served a shorter term and is notable for his service during World War II. Captain Fitzpatrick does not hold significant historical prominence in terms of command tenure or influence comparable to that of Henderson. Thus, Henderson's lasting legacy and substantial influence in shaping the modern Marine Corps contribute to why he is recognized for the longest tenure in that role.