Well, that's a first. Luckily I was able to contact the author to ask him about the aliens in the background. For Legend fans it looks like a good setting for converting to Mercenary Breed, as it is a more interesting background than the Argo Galaxy.
"Desperate for resources, humanity colonised the solar system, terraforming the other planets where it could and setting up huge artificial environments where it could not. After a number of false starts humanity developed effective star travel and sent out starships to explore the nearby stars in earnest. Though the development of the stardrive was expensive and time-consuming, pressures from a planet almost depleted of resources and gasping for room meant few options were left.
After years of fruitless search for extra-terrestrial life, the accepted theory was that humanity was alone the universe, separated from any other interstellar civilisation by the millions of years of history. Imagine the surprise of the first explorers when they encountered not just one, but many other races exploring the stars. Some were humanoid, others were totally unlike anything humanity had ever seen; some were friendly, others were hostile in the extreme.
Despite the hope of the optimists, the contact led to an increase in the already-existing conflict for land between the oxygen-breathing races of the galaxy. Scuffles for resources had already grown into a state of continual, low-key war, governed and complicated by numerous local treaties between dependent colonies, free worlds and the core systems of each species. Furthermore, each free world felt able to deal with other free worlds, whether to declare hostilities or to declare alliances. The presence of the ambitious and flexible humans of the United Nations of Earth (UNE) aggravated this cold war.
Whilst they lost the early encounters with the more advanced races, later encounters were frequently won by the more adaptable humanity, leaving the older species concerned that they would lose their status in the galaxy. Rather than ally against the UNE – an act which would be impossible for some species anyway – the older species attempted to tie humanity's hands with a treaty that they believed would undo the gradual dominance of the UNE.
The agreement, called the Sirius Treaty by the UNE, states that outright war and weapons of mass destruction are outlawed. It limited the use of technology on primitive planets, attempting to protect those civilisations from exposure to influences that could readily destroy them. Slavery, such as that of forced, unpaid colonies is abolished. There is more, but the key is that violators are denied access to the planets on which they violate the treaty, or are dealt with by a new, independent force set up to monitor the treaty: the Invigilators.
The Invigilators are cybernetically enhanced sophonts using the best technology the treaty species can supply. This peace-keeping force was given resources and technologies to make them independent of the other species and able to function as a self-contained force. Though totally committed to the tenets of the treaty, the Invigilators quickly adapted their role to monitor interaction and development on newly-discovered planets, ensuring the local development of technology is not affected by interactions with off-world technology.
Though it successfully limited outright, interstellar war with the devastating, high-tech weapons available to the starfaring species, the treaty brought even further chaos. The UNE, humanity’s Core Worlds, and the MetaCorporations rapidly founded new institutions to fight limited war. The almost indolent, older species quickly found themselves having to adjust to keep up.
Non-Interference? The UNEST Legions...
The principle of non-interference embodied within the treaty forbids the use of any technology higher than that in general use on a protected world. No advanced species is permitted to give less-advanced species new technology: the primitives must make their own discoveries. Anachronous technology cannot even be taken on-planet.
Critics have already pointed out that this is likely to 'freeze' development. Several situations have already arisen where the locals have developed their own answers to problems using inventions that were possibly inspired by off-world technology. But the Invigilators acted swiftly, according to their directives, and suppressed the technology, wiping out those who committed the infraction. However, the treaty races quickly established that using precedents from their own past avoided the attentions of the Invigilators completely: the technology, in-period, was justified.
This became a gift to the UNE. With its long and intensely varied history of war, humanity merely looked at its past and found the means to obey the letter of the treaty – if not its spirit. From the Romans to the Mongols, from the English longbowmen to the Samurai, Earth had a plethora of highly effective models, in a wide range of technologies on which it could base a new type of soldier. The UNEST-SF were created, able to adapt to any historical circumstance with the best humanity could offer.
The UNE forces are already given the best implants earth could provide. To a Star Marine, tactical datajacks, combat nanofactories, subcutaneous comlinks and subdermal armour were a necessity. But when that same star marine picks up a Roman sword, dons Roman armour and joins a legion of the UNEST, a new breed of warrior is born. He becomes the special forces legionnaire, a soldier wearing and carrying equipment that looks just like that of their historical counterparts, but each item of equipment is the result of the best GM, ceramic and and metallurgic technology earth can build. And with his cybered upgrades being an invisible, intrinsic part of himself, he cannot be accused of taking illegal technology down to the planet’s surface.
This is a soldier of the UNEST-SF. Though armed with a sword, his subdermal armour, hidden comlinks and cybernetic implants make him a potent adversary. And he needs all the advantages he can get when he faces the duplicitous species of the Sirius Treaty, and the might of the Invigilators!"