Star Trek: Enterprise season 3 introduced the military organization MACOs (unofficially, space marines). First airing in 2001, Star Trek: Enterprise documented Earth's initial first contacts and voyages into deep space. Echoing significant societal events of the time, Enterprise season 3 took on a noticeably darker tone. Transitioning from standalone episodes to a complex and weighty season-long serial arc, a devastating attack on Earth by the mysterious Xindi landed Captain Jonathan Archer (Scott Bakula) with a heavy responsibility for establishing contact and preventing (or intercepting) further attacks. Anticipating a need for the presence of a conflict-trained unit, Archer requested a military assignment to the NX-01 Enterprise.

https://screenrant.com/star-trek-enterprise-maco-space-marines-explained/