This merc group was the brainchild of a former JSDF member and a former CIA special forces operative in the late 70s(although it was under a different banner but the group mentality was much the same).
They put the group together based not any adherence to ideology beyond that of supporting the individual soldier in their employee, they despised the system of Nation States, They took work from both halves of the iron curtain, Both of them more or less despised the system that tossed aside individual soldiers for political expediency, for example Big Boss himself was labeled a terrorist back during incident at San Hieronymo when his former CIA unit went off the reservation, he ultimately was acquitted after ending the incident, but after that he was more or less tossed under a bus.
They fought the CIA and KGB in the Caribbean during the late 70s, Ultimately establishing themselves an unofficial nuclear power when they obtain a secretly delivered CIA nuclear warhead for an experiment they were conducting in Costa Rica at the time. I mean when i think private army? I think Diamond Dogs, i think MSF and i think Outer Heaven, they had warships, gunships, transport helicopters, tanks, land vehicles, basically picture all the standard affair you would expect from a regular army, and then combine it with Research and Dev, Logistics, supply lines and all that, they basically offered everything from personal military contracting to the development of weapons.
It made them a scary amount of money, it ended but it was reborn again after a...
well an incident, you would need to dig into the MG lore to understand i think, but i think i gave an adequate picture of just what Outer Heaven was, Eventually it became its Nation, but that's a different story. But during the late 80s and early 90s it went from a PMC into a full fledged world power.