From: The Post-UN Era. Williamson, Mike. Baen Historical Press, Heinlein, 2555.
As you have seen in preceding chapters, the attempt by the United Nations to secure the Colonies and secure control over the entire Human Sphere was terminated when Fleet — then called the United Nations Peace Force — launched a coup against its superior officers. The then-Captain John Walker believed that the war was beyond being won and, in fact, was heading rapidly towards mass slaughter on both sides. Be that as it may — and research on Earth suggests that the UN intended to resort to weapons of mass destruction again — his successful coup against the United Nations broke their power completely. The Treaty of Unity, rammed down the throats of Colonies and Earth alike, secured peace, at a price.
The UN had held hundreds of worlds in its grip, but now that grip was broken. Some worlds, like Terra Nova, collapsed into chaos, ironically secured by members of the former United Nations Infantry, who were no longer welcome back home. Others, like Heinlein, Williamson’s World and New Washington, managed to regain control quickly and even assisted in the reparation of the Infantrymen to other locations. Some even became citizens of the worlds they had tried to suppress.
And Fleet had secured control of space. The remains of the UNPF was still the strongest force operating in space and John Walker dictated peace terms that included a firm guarantee that Fleet would remain supreme, a neutral and disinterested force that would ensure that interstellar war would no longer threaten humanity. The Fleet was, to all intents and purposes, the ruler of the Human Sphere, although it ruled with a very light hand. Fleet was denied any major ground combat element and the vast majority of the former UN Infantry was disbanded. It would never be able to mount a campaign on a planet’s surface. Indeed, the treaty even allowed the development of space-based defences, everything, but jump-capable warships.
The remainder of the UN Infantry were in an unfortunate position. Some units returned to Earth and were disbanded. Others took service with various planetary governments, or were encouraged to settle their own worlds, along with the teeming refugees from Earth. And some became soldiers for hire. Five years after Fleet mounted its coup, there were hundreds of mercenary units operating in the Human Sphere. Some were tiny, composed of only a few hundred men, and some were large enough to qualify as armies in their own right. The most famous of these was the Legio Exheres, the Legion of the Disinherited.