Marine Detachment or MarDet was a unit of 35 to 55 United States Marines aboard large warships including cruisers, battleships, or aircraft carriers. They were a regular component of a ship's company since the formation of the United States Marine Corps up until the 1990s. Missions of the Marine Detachment evolved over time, and included protecting the ship's captain, security and defense of the ship, operating the brig, limited action ashore, securing nuclear weapons and ceremonial details.
Marines served aboard sailing ships as a small amphibious force able to capture and hold minor port facilities as required for protection of American interests. Marine sharpshooters were often stationed in the rigging during ship-to-ship combat to fire at officers and helmsmen aboard enemy warships. Marines often operated naval artillery during general quarters when the distances of gunnery engagements exceeded the range of small arms.
Each MarDet included a Marine Corps commanding officer who reported to the Commandant of the Marine Corps through the ship's captain. When more than one Marine officer was assigned to a ship, United States Navy Regulations required one Marine officer to be aboard ship at all times unless excused by the ship's captain. Marine officers below the rank of major sometimes served as officer of the deck.
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- Mayo, Claude Banks (1939). Your Navy. Los Angeles: Parker & Baird Company. pp. 311–317.