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The Marines are the seaborne military forces of the World Government, tasked with maritime law enforcement and naval warfare operations. They maintain a presence on all of the world's known seas, through countless branch offices; however, the vast majority of their forces are focused on the Grand Line, in which resides their base of operations, Marine Headquarters. Collectively, they stand as one of the Three Great Powers, alongside the Four Emperors and the Seven Warlords of the Sea, who were later replaced by the SSG.
Due to their actions and role, they are one of the major antagonist groups of the series overall, specifically they serve as the main antagonists in the Romance Dawn, Loguetown, Enies Lobby, Sabaody Archipelago, Marineford, Return to Sabaody and Egghead Arcs. They are also antagonists in various non-canon material, such as the Warship Island, Goat Island, G-8 and Marine Rookie filler Arcs, the fifth and eleventh movies, and the third, sixth and tenth specials.
The Science Unit (科学部隊, Kagaku Butai?) is the Marines' scientific research and development team, consisting of scientists and led by Dr. Vegapunk, the world's smartest man, with his bodyguard Sentomaru acting as captain. The science team is primarily responsible for most of the special technology found in the hands of Marines
The Giant Squad (巨人部隊, Kyojin Butai?) is a Marine group composed solely of giants. During the Summit War of Marineford, they were assigned with the duty of protecting the execution platform holding Portgas D. Ace.
SWORD is a secret unit within the Marines consisting of publicly resigned Marines, apparently led by X Drake. Despite their status, they can still operate on the Marines' behalf and are not beholden to orders from higher command, but are considered expendable should their actions prove troublesome.
The Marine Bureau of Investigation (海軍犯罪捜査局, Kaigun Hanzai Sōsa-kyoku?, English: "Naval Bureau of Investigation") is a Marine division in charge of investigating and collecting data of major incidents happening within their jurisdiction. They are led by Tensei, the division's director.
The Communications Department (通信部, Tsūshin-bu?) is a Marine division in charge of detecting, intercepting and tracing Den Den Mushi calls from outlaw organizations, such as the Revolutionary Army.[18] This division has existed at least since twelve years ago, as it was how the organization became aware of the Revolutionary Army's victory at Tumi.[19]
The Marines' Photography Department (海軍写真部, Kaigun Shashin-bu?) is the Marine unit responsible for taking pictures of pirates and other criminals to utilize in wanted posters. They were once led by Attach, until he got fired for frequent mistakes.