In this world, nations once came together and decided that global empires are generally a bad thing for the species as a whole - since their effect on history and culture is very similar to monopoly's effects on the economy. They also act like focuses of the attention, with everybody ambitious beginning to just want to either rebel against the empire or to overthrow the emperor and rule it themselves.
So they came up with an international organization whose purpose is to keep the governments in check and prevent them from gaining a too large military or economical advantage over other countries, by force, if necessary. This organization is itself has a large military, manpower, and has a political influence on the countries, so they, theoretically, can take over the entire world with relative ease. But they stay true to their mission, only taking action to level out the playing field for everybody else (either by smacking those who got too greedy and imperialistic or by bootstrapping those who fell too far on the other side of the scale) and other than that let the governments of the world do whatever they want (both inside their countries, and to each other).
Basically, they're a geopolitical antimonopoly committee with executive power and a dash of humanitarian aid mission.
Now, the question is, what are the mechanisms that can prevent such an organization from becoming infiltrated or corrupted, and just deciding to conquer everybody themselves? At least on a timescale of one or two centuries, but preferably the more future-proofed they are, the better.
It should preferably be some logical element of the organization itself and not something immutable that itself enforces its will on the organization - like an immortal CEO, or mind-affecting magic, or something like this.