- This article contains spoilers.
Militaires Sans Frontières (French for Soldiers Without Borders) was a group led by Big Boss. They were stationed in Colombia when they were approached by Gálvez and Paz, representing Costa Rica, and were asked to assist the "nation without a military". In return, Gálvez offered the group a off shore plant to use as a base off the coast of the Caribbean, which was dubbed Mother Base. After these events, the group would come to be known as the founders of "Outer Heaven."
Big Boss doesn't seem to consider MSF as a mercenary group, and even feels offended when it is considered to be as such, as shown when Big Boss had protested to Galvez for assuming they were just a pack of "Dogs of War" when Galvez tried to hire them. The Militaires Sans Frontieres, while undergoing Operation Peace Walker, also dispatched several of its forces into many battlefields for income to increase its numbers as well as feed its men (referred to as Outer Ops).
- According to Kaz, during his call to Cipher shortly before Paz stole Metal Gear Zeke, the Militaires Sans Frontieres were going to be dispatched to Angola for their services. Also, Cipher planned to have Big Boss and the Militaires Sans Frontieres act as the deterrent force for their new world , but after Big Boss refused the offer, they then tried to frame the Militaires Sans Frontieres by having their agent Pacifica Ocean use Metal Gear Zeke to launch a nuclear strike on the East Coast and frame the Militaires Sans Frontieres as being an extremist cult. However, that plan failed as well when Big Boss defeated Zeke and Pacifica.