Military Assault Command Operations, the United Earth combat force whose soldiers joined Enterprise NX-01 in 2153 to provide firepower during the mission into the Delphic Expanse.
The MACOs are soldiers of Earth's military, distinct from Starfleet. After the Xindi attack of 2153, Captain Archer requests a detachment for the search in the Expanse; they join the ship before it sets out and are first seen in The Xindi, under Major J. Hayes.
Their presence creates friction. Hayes and Lieutenant Malcolm Reed clash over responsibility for the ship's security, a rivalry that boils over in Harbinger, but the troops prove vital in boarding actions and raids throughout the third season. Hayes dies rescuing Hoshi Sato in Countdown.
MACOs stay aboard in the fourth season and appear in the mirror universe of In a Mirror, Darkly. Star Trek Beyond reveals that its villain, Balthazar Edison, was a MACO officer before the Federation was formed.