After leaving the Patriots in 1972, Naked Snake, a.k.a. Big Boss, formed the Militaires Sans Frontières to provide military force to anyone who needed it, regardless of nation or ideology.

As such, the MSF specialized in providing a full range of military services, including combat, logistics, training, weapons, outfitting, and R&D. In addition, the MSF was designed to merge the small footprint and exceptional performance of special forces groups with the full military might of a full regular army, as a means to fully break free of nation-states.

Employed by the Colombian government sometime after 1972, Snake and the MSF fought an opposing mercenary unit led by Kazuhira Miller for their last contracted job. After MSF defeated the enemy force, Snake offered Miller a place in the group, which he eventually accepted, later becoming its second-in-command.

MSF and Mother Base ended up attacked by XOF.Of the soldiers present on Mother Base at the time of the attack, only Miller, Big Boss,, the medic and Mosquito survived both the attack itself and the chopper crash shortly thereafter. And of them, only Miller managed to survive with comparably lesser injuries, as Big Boss fell into a coma, and the medic was hit with shrapnel to the head. The Medic was then put into a medically-induced coma and gradually transformed into Venom Snake, a body double for Big Boss, at Zero's request. Huey had also escaped via chopper by XOF, which also led Miller to bore a grudge against him due to it becoming apparent that he set them up. He would later spend a lot of time trying to track Huey down.

-Snake/Big Boss
-Kazuhira Miller
-Huey Emmerich
-Strangelove
-Paz Ortega Andrade
-Cécile Cosima Caminades
-Amanda Valenciano Libre
-Chico
-Miel
-Soldiers

There were five divisions within the Militaires Sans Frontières, at least during their time at the Mother Base:

The main fighting/sneaking force of the Militaires Sans Frontières and source of income.

Responsible for enhancing or otherwise creating weapons and equipment.

Responsible for providing the food for the Mother Base which affects morale. It was formed by Kazuhira Miller shortly after Big Boss encountered a LAV-typeG unit at Bananal Fruta de Oro as a precautionary means of provisions for the troops.

Responsible for the healing of any personnel requiring medical attention due to sickness, injury or PTSD. They are also involved with R&D in regards to the development of weapons with various ways of knocking out personnel . The idea of a medical team for the Mother Base's Sick Bay was also planned by Kazuhira Miller after Amanda was airlifted to Mother Base wounded with a broken leg, although a Medical Team for MSF itself existed long before then.

Responsible for providing intel for the Militaires Sans Frontières in regards to enemy activity or a weakness of the enemy that could be exploited as well as offering support from Mother Base for MSF soldiers on field missions. The Intel Team also often worked with the R&D Team to develop various radars for MSF, and could also deliver supplies to allied mercenary units. It was brainstormed by Kazuhira Miller shortly after Chico ended up rescued. It was also implied that the Intel Team, or a similar unit called the MSF Intel Corps., were responsible for the recording/adapting of various old data files for the MSF.