federal agent of the Diplomatic Security Service, known as the Regional Security Officer (RSO) who is the senior U.S. law enforcement representative and security attaché at U.S. diplomatic posts around the world. In addition, MSGs provide security for visiting American dignitaries and frequently assist the RSO in supervising host country or locally employed security forces that provide additional security for the exterior of embassies. The MSGs fall under operational control of the RSO and are administratively controlled by the Marine Corps Embassy Security Group. The secondary mission of Marine Security Guards is to provide protection for U.S. citizens and U.S. Government property located within designated U.S. Diplomatic and Consular premises during exigent circumstances, which require immediate aid or action. MSGs focus on the interior security of a diplomatic post's buildings. In only the most extreme emergency situations are they authorized to provide special protection to the senior diplomatic officer off the diplomatic compound. MSGs carry a certain level of diplomatic immunity in the performance of their official duties. Organization The Marine Security Guards number approximately a thousand Marines at 174 posts (also known as "detachments"), organized into nine regional MSG commands and located in over 135 countries in 18 time zones, as well as its headquarters at Marine Corps Base Quantico. Headquarters Company, along with MSG School, is composed of approximately 100 Marines providing administrative, logistical, legal, training and education support. The remaining nine regions are commanded by a lieutenant colonel, and typically entail a number of detachments in several countries. The companies are as follows: Each Detachment is commanded by a Staff Non-Commissioned Officer, being one of the few instances where an enlisted Marine may hold the title of "commander". Generally between the ranks of Staff Sergeant and Master Gunnery Sergeant, Marine Detachment Commanders serve two tours, which generally last 18 months each. Unlike their subordinates, however, Detachment Commanders may be married. The minimum detachment size is seven MSGs (Marine Security Guards) and a one detachment commander. This allows for posts to be manned at all times while allowing each of the Marines to conduct other routine training, internal management of the detachment and have some time off. A Marine Security Guard usually serves three 12-month tours of duty. Marine Security Guard "watch standers" are enlisted Marines from the rank of Private First Class to Staff Sergeant. Duty After every three years as a Marine Security Guard with the