(covers information from several alternate timelines)
"Captain Archer said he wanted the best."
Military Assault Command Operations (commonly abbreviated as "MACO", pronounced "MAY-ko") was a United Earth organization that existed as a part of the United Earth Military during the 2150s, prior to the founding of the Federation. The MACOs were not associated with Starfleet. As of 2153, MACO tactics and technology were two to three years more advanced than those of Starfleet, but their combat expertise was initially limited to simulations conducted on Earth. (ENT: "Harbinger"; Star Trek Beyond)
The military officer who oversaw the MACOs was General Casey. (ENT: "The Expanse") MACO posts included Atlanta, Georgia and the Janus loop. Training posts included West Point in New York, as well as Jupiter Station for zero gravity/rifle training, and Luna for survival training. (ENT: "The Xindi", "Anomaly", "Harbinger", "Hatchery", "The Council")
A detachment of troops commanded by Major J. Hayes was assigned to the Starfleet vessel Enterprise NX-01 for a mission into the Delphic Expanse in 2153. Admiral Maxwell Forrest was surprised that Captain Jonathan Archer had requested military officers aboard his ship. (ENT: "The Expanse") When the MACOs first came aboard Enterprise, they certainly felt like outsiders, tasked to integrate with the ship's Starfleet contingent. (ENT: "Countdown") The MACOs saw their first action six weeks into the mission; in September, Hayes and a six-man team rescued Archer, Commander Tucker, and Kessick from the clutches of a trellium mining planet. (ENT: "The Xindi") By the end of their time in the Delphic Expanse, none of the MACOs aboard Enterprise felt like outsiders anymore. (ENT: "Countdown")
Following the foundation of the United Federation of Planets, the organization was disbanded, with personnel being offered Federation Starfleet commissions on a discretionary basis and possible offer commissions in other agencies in the Federation. The Federation Starfleet, just as with its United Earth predecessor, continued to distance itself from the notion of being a military organization, causing deep conflict for at least one former MACO who donned a Starfleet uniform. (Star Trek Beyond)
- Rescuing Captain Archer and Commander Tucker from a Trellium mining planet (ENT: "The Xindi")
- Trying to fend off an Osaarian boarding party (ENT: "Anomaly")
- Battling on the Loque'eque homeworld (ENT: "Extinction")
- Trying to fend off a Xindi-Reptilian boarding party (ENT: "Rajiin")
- Providing escort for Enterprise officers on the Seleya (ENT: "Impulse")
- Providing escort for Enterprise officers on the Xindi-Arboreal colony (ENT: "The Shipment")
- Providing backup on a Human colony in the Expanse (ENT: "North Star")
- Guarding the prisoner Degra (ENT: "Stratagem")
- Providing combat training for Enterprise officers (ENT: "Harbinger")
- Containing a Sphere-Builder test subject (ENT: "Harbinger")
- Raiding an Illyrian vessel (ENT: "Damage")
- Providing escort for Enterprise officers inside a Delphic Expanse sphere (ENT: "The Council")
- Recovering Ensign Hoshi Sato from a Reptilian vessel (ENT: "Countdown")
- Assisting in destroying the Xindi weapon (ENT: "Zero Hour")
- Repelling a Sphere-Builder boarding party (ENT: "Zero Hour")
- Accompanying Archer to a meeting with Vosk on 1944 Earth (ENT: "Storm Front, Part II")
- Escort and guard for the prisoner Arik Soong (ENT: "Borderland")
- Attempt to fend off an Augment boarding party (ENT: "Borderland")
- Attempting to take back Cold Station 12 from the Augments (ENT: "Cold Station 12")
- Escort and guard of Tellarite delegation (ENT: "Babel One")
- Boarding the Romulan drone ship (ENT: "Babel One")
- Attempting to fend off a Klingon Augment boarding party (ENT: "Affliction")
- Ambush of kidnappers on Rigel X, just prior to the formal treaty signing that created the United Federation of Planets (ENT: "These Are the Voyages...")
|Militaries of Earth|
|Confederate States:||France:||Germany:||Japan:||Old Britain:|
|Army||French Guards||Wehrmacht (Luftwaffe) • SS||Navy||Navy • Air Force|
|United States of America:|
|Army (Army Air Forces) • National Guard • Navy • Air Force (Air Defense Command; Air Materiel Command; Strategic Air Command) • Marine Corps (MARDET)|
|United Earth: United Earth Military (Military Assault Command Operations)|
- "The Xindi" (Season 3)
- "The Shipment"
- "North Star"
- "Chosen Realm"
- "Doctor's Orders"
- "The Forgotten"
- "The Council"
- "Zero Hour"
- "Storm Front, Part II" (Season 4)
- "Cold Station 12"
- "The Augments"
- "Babel One"
- "These Are the Voyages..."
- Star Trek films
- Star Trek Beyond (mentioned)
The MACO Enterprise mission patch depicted a mako shark. ("Countdown" text commentary, ENT Season 3 DVD) The mission patch was designed by Scenic Art Supervisor Michael Okuda. (Star Trek: Costumes: Five Decades of Fashion from the Final Frontier, p. 217)
The patch was later used during the production of Star Trek Beyond, modified to feature the name of the USS Franklin rather than Enterprise. The MACO patch was stenciled on the sides of PX70 motorcycles used in the film  and was printed onto T-shirts given to the cast and crew. 
Regarding the disbanding of MACO, Star Trek Beyond co-writer Doug Jung once noted, "I thought it'd be kind of interesting [….] You can make some modern analogies in terms of like the CIA, which is – you're asking these people, you're training them their whole lives to see the world one way, and then you say, 'Oh, by the way, you're now obsolete, but we'd like you to do [this other thing].' It's tough." 
The novelization of The Expanse states that Captain Archer was initially uncomfortable with the notion of military personnel serving aboard Enterprise but reconciled himself to the idea, given that it seemed as if the MACOs would be beneficial to the search for the Xindi weapon. The same novelization also suggests that MACO be pronounced "Mah-co" rather than "May-co", despite the latter being the commonly used canon pronunciation.
In the Romulan War Pocket ENT novel Beneath the Raptor's Wing, the "O" in MACO stands for "organization" instead of "operations", making the MACOs' proper title "Military Assault Command Organization".
In Star Trek Online, the MACOs are an internal branch of Starfleet focused on infantry and ground combat operations. MACO personnel use standard, naval-derived Starfleet ranking structure,rather than the army/marine style. They first appear in-game alongside the Klingon Honor Guard as part of Task Force Omega, a specialized cross-faction unit formed to fight the resurgent Borg Collective, and MACO-affiliated characters appear in other ground-based tactical scenarios further into the story, usually in sections featuring combat against enemies such as the Voth, Vaadwaur, and Tzenkethi. Starfleet-aligned player characters can obtain specialized MACO armor and weaponry, as well as cosmetic player titles such as "MACO Commander", implying that MACO membership is more of a subclass of Starfleet officer than an independent organization.