This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
The Emerald Chain was a capitalist syndicate of Orions and Andorians, though it also included many other species. Described as a "federation of mercantile exchanges", it rose in the wake of the Burn in the 31st century and achieved the power of a de facto state. (DIS: "Die Trying", "There Is A Tide...")
In 3189, the Emerald Chain operated on the basis of a charter. It was led by an Orion woman named Osyraa, who held the title of Minister, and by a Congress empowered to amend its charter. (DIS: "There Is A Tide...")
The Emerald Chain controlled numerous mercantile exchanges, such as the Mercantile on Hima, where courier and other commercial transactions took place. They also ran a private salvage yard on Hunhau, operated by debt slave laborers working under brutal conditions. (DIS: "That Hope Is You, Part 1", "Scavengers", "The Sanctuary")
The Emerald Chain arose in the wake of the Burn in the late 31st century. Flouting the Federation's Prime Directive, they made reckless first contacts with pre-warp civilizations such as the Kwejian and extracted valuable resources from them with predatory deals. (DIS: "The Sanctuary")
In spite of the brutality of the Emerald Chain's capitalist society, it also maintained the best-funded and equipped scientific and research facilities in the known galaxy. At least some of its subjects viewed the Chain's sentient rights abuses as being unfortunate necessities of life in a galaxy of scarce resources rather than as criminal acts. (DIS: "There Is A Tide...")
In 3189, the Emerald Chain was pushing aggressively, bringing them into conflict with the Federation. Starfleet was tracking fifty star systems that had been ensnared by the Emerald Chain, most of which were expected to collapse, but lacked the ships to intercede. The USS Cuyahoga reported Orion and Andorian vessels from the Emerald Chain in the Sigma Draconis system. Three weeks later, Starfleet obtained intelligence about an imminent move on Argeth, shortly before the Emerald Chain broke off negotiations. (DIS: "Die Trying", "Scavengers", "The Sanctuary")
Later that year, Minister Osyraa forced Federation Starfleet Commander in chief Charles Vance to come to the negotiating table by hijacking the USS Discovery and holding its crew hostage. Minister Osyraa proposed an armistice as part of an eventual plan to obtain popular legitimacy for the Chain by uniting with the Federation -- thereby sharing in the Federation's continued reputation for hope and obtaining tacit Federation support for their brand of capitalism. She also sought this union in the hopes of obtaining and replicating Federation spore drive technology.
Steps she proposed towards this goal included having the Federation government openly sanction the century-old trade between Starbase Deep Space 253 and an Emerald Chain mercantile exchange, and the opening of an Emerald Chain Embassy aboard the station. On her end, Osyraa indicated she was willing to make significant territorial concessions by pulling out of vassal worlds, and had expanded significant political capital to have the Emerald Chain Congress amend its charter to ban slavery. Fleet Admiral Vance, acting on behalf of the Federation President, rejected Osyraa's offer when she made it clear that the Emerald Chain's representative in Federation matters would only be her proxy, and that she would refuse to stand trial for the sentient rights abuses committed during her time in power. (DIS: "There Is A Tide...")