Marine Guards, formally members of the Marine Corps Embassy Security Group (MCESG), are U.S. Marines detailed to the Department of State to provide internal security at diplomatic and consular posts. The program was formalized by a memorandum of agreement between the Department of State and the Department of the Navy in 1948, following congressional authorization under the Foreign Service Act.
Their primary mission is the protection of classified information and equipment, with a secondary mission of protecting U.S. citizens and government property at the post. Marines staff Post One, the main security checkpoint inside a chancery, controlling access and monitoring alarms. They do not perform external perimeter defense, which is the responsibility of the host nation under Article 22 of the Vienna Convention on Diplomatic Relations (1961), requiring receiving states to protect mission premises.
Marine Security Guards (MSGs) operate under the authority of the Regional Security Officer (RSO), a Diplomatic Security Service special agent, and ultimately the Chief of Mission. Their rules of engagement are governed by the MSG handbook and post-specific orders, with lethal force authorized primarily for the defense of life.
Key historical episodes have shaped the program: the 1979 seizure of the U.S. Embassy in Tehran, the 1983 Beirut embassy bombing, the 1985 Moscow embassy spy scandal involving Sgt. Clayton Lonetree, and the 1998 East Africa embassy bombings. Each prompted reforms in staffing, training, and physical security standards under the Secure Embassy Construction and Counterterrorism Act of 1999.
Marine detachments typically number 6–35 Marines depending on post size and threat level, led by a Detachment Commander (a Staff NCO). Several other states maintain analogous services — for example, the UK uses contracted Military Provost Guard Service or private security at some missions, while France deploys Gendarmerie personnel.
Example
During the 2012 attack on the U.S. diplomatic compound in Benghazi, the absence of a Marine Security Guard detachment at that facility became a focal point of subsequent congressional inquiries.