Most civilians don’t realize this, but the Enterprise actually carries a full contingent of Starfleet Marines -- for combat operations, as well as added ship defense. In fact, all Starfleet ships have at least a few stationed aboard – you never know what dangers await us out in space, or what hostile forces might show up during an away mission. Starfleet security maintains all safety and police matters aboard a ship, while the Marines deploy with – or ahead of -- away teams, and handle all ground hostilities on a planet.
Marines have been a common part of every Starfleet crew since the early MACO days aboard the Enterprise NX-01. The Marines are a branch of Starfleet, but they have their own command structure. This means that most Marine officers graduate from Starfleet Academy, but go on to additional command training before being deployed. The majority of the Marines, however, consist of enlisted personnel – trained soldiers who dedicate their lives to keeping the Federation safe and free.
-Lieutenant Sam Archer
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June 17, 2364 -- (Original Devron Timeline)