From the U.S. embassy website:

Marine Security Guard Detachment

The mission of Marine Security Guards is to provide protection to mission personnel and prevent the compromise of national security information and equipment at designated diplomatic and consular facilities.  Marine Security Guards are prepared to execute plans for the protection of the mission and its personnel as directed by the chief of mission or principal officer through the Regional Security Officer.


I remember seeing Marines at the U.S. Embassy in The Bourne Identity and wondering what their duties were, if any, outside of standing guard because that would be boring as hell. I hated standing fire watch or any other guard duty that kept me in one location for extended periods of time. Working at an embassy would be very different than the kind of duty I pulled though.

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