The United States Marine Corps was a military corps branch of the armed forces service of the United States of America, reporting to the US Department of the Navy.
The Marines were used as naval infantry, specializing in fighting on ships and conducting amphibious assaults, and coordinated air/land/sea assaults.
In 2364, the Q-entity took the form of a 1980s Marine captain to lecture Jean-Luc Picard on the barbarity of Human history. (TNG: "Encounter at Farpoint")
Q's rank insignia were visible on his jacket, which appeared to be modern to the time the episode was filmed, in 1987.
Real word Star Trek-affiliated staffers who had served in the Corps, included Star Trek: The Original Series Writer/Producer Gene L. Coon during World War II, as well as reference author Stephen Edward Poe as a Marine Corps pilot and Intelligence Officer after the war. (Inside Star Trek: The Real Story, 1997, pp. 401-402)