What is the rank of the individual who leads a MEF?

The individual who leads a Marine Expeditionary Force (MEF) holds the rank of Lieutenant General. This rank is typically responsible for large-scale command structures such as a MEF, which is the largest type of Marine Air-Ground Task Force (MAGTF). A Lieutenant General commands multiple units and has the authority and responsibility to oversee significant operations and strategic planning. This rank is appropriate for the complex nature of leading a MEF, which includes various components such as ground, air, and logistics services within its operational scope.

The other ranks, such as Major General and Colonel, do lead smaller formations within the Marine Corps, but they do not have the command authority needed to oversee an entire MEF. The usage of "Sir" does not refer to a rank and is a form of address rather than a specific level of command. Thus, the rank of Lieutenant General is distinctly suited for the leadership role within a Marine Expeditionary Force.