Starfleet Marine Corps
From Star Trek: Theurgy Wiki
The Starfleet Marine Corps, or simply the Starfleet Marines, were the infantry forces of the Federation's Starfleet. They existed as a sub-section of Starfleet Security, and they rarely served actively aboard starships since their duties were limited to planetary defence.
Unlike the rest of Starfleet's service personnel, who used a naval ranking structure, the Marines used a rank structure based on equivalents of Earth infantry-based militaries. Early variants of the SFMC was the Starfleet Ground Attack Marines of the 2160s and MACOs. (TNG novel: The Forgotten War; FASA RPG modules: Cadet's Orientation Sourcebook, Federation Ship Recognition Manual, Star Trek IV Sourcebook Update, The Four Years War)
During the Four Years War (c. 2250s), Starfleet Marines landed on Laxala, and constructed plasteel huts for their bases.
In 2293, the SFMC was slated to take part in Operation Retrieve. They would have landed teams on Rura Penthe and participated in a direct assault on the Klingon prison complex on that planet. (TOS movie: Star Trek VI: The Undiscovered Country. In this film, the SFMC was not spoken of in dialogue, however the Operation Retrieve command briefing did feature them. This marks the only canonical mention of the SFMC.)