The Inner Sphere is a region of interstellar space surrounding Earth to a radius of roughly 450 - 550 light-years, generally demarcated by the outer borders of the "Great Houses." Within this region of about 2 million stars, there are approximately 2000 inhabited planets. Beyond the Inner Sphere is the Periphery.
Colonization of the Inner Sphere began in 2108, when the Terran Alliance launched the first manned faster-than-light mission aboard the ''TAS Pathfinder''. Between 2108 and 2235, the Terran Alliance colonized and ruled the first centralized interstellar government in human history, controlling more than 600 colonies within 120 light years of Terra. However, corruption and logistics made it impossible to adequately govern such a large body. Private concerns "illegally" funded exploration and colonization of Inner Sphere worlds, as well as worlds located in the Periphery and the Deep Periphery. Despite the best efforts of the Terran Alliance to end "blockaderunning", by the end of the Outer Reaches Rebellion, the Terran Alliance was declining.
From c. A.D. 2300 to 2787, the Inner Sphere was divided into five "pie wedges" around a central, roughly circular core:
Shortly after the formation of the Great Houses, the Age of War began. The end of the Age of War accompanied the formation of the Star League, a multinational administrative and peacekeeping treaty, which lasted for ~250 years. During the Star League, mankind achieved technological marvels, and the relative stability of the age is still viewed as a utopian period by the people of the Inner Sphere; though the stability of the Star League may be more myth than legend. With the collapse of the Star League in 2781 and exodus of the Star League Defense Force from the Inner Sphere, the Great Houses consumed the moribund Hegemony and turned on to each other. Only Terra remained independent under the aegis of neutral interstellar communications agency, ComStar.
During the next 250 years, civilization was hammered from its 28th Century peak down to 21st Century technology on many worlds and to more primitive states on others; and hundreds of worlds had their populations exterminated all together. The cause for this warfare was a matter of feudal succession to the throne of the fallen Star League hence the four wars of this period were called Succession Wars, while the five Great Houses were said to rule Successor States.
Despite hundreds of billions of deaths, the first three Succession Wars resulted in little change to the map after the First Succession War divided the Terran Hegemony between the five Successor States.
The brief Fourth Succession War brought a dramatic change when the Lyran Commonwealth and Federated Suns, on opposite sides of the Sphere, united at the expense of the other Successor States' territories. A number of minor nations appeared in the Inner Sphere during or shortly after the Fourth Succession War, at the expense of the Draconis Combine, the Free Worlds League and the Capellan Confederation.
The political balance of the Inner Sphere changed fundamentally when lost scions of the Star League Defense Force, the Clans, returned to conquer the Inner Sphere and restore the "Star League". The Clan Invasion (3049-3052) nearly destroyed the Free Rasalhague Republic, while the Lyran half of the Federated Commonwealth and the Draconis Combine lost many systems. Though the Clan invasion was halted after the Battle of Tukayyid, the Invading Clans had gained a foothold within the Inner Sphere. As denizens of the Inner Sphere, Clan politics and practices tended to change, reflecting both an increase in affluence (the Clan homeworlds are considered marginally hospitable compared with worlds of the Inner Sphere) and the need to interact with their new neighbors.
The Periphery surrounding the Inner Sphere is home to a few nations of moderate power and many independent worlds cut off from civilization with the collapse of the Star League. The Inner Sphere perceives the Periphery much like modern developed nations perceive the Third World, though some of the major Periphery nations compare well with the weaker Inner Sphere states.
Beyond the Periphery is unexplored space (sometimes called the Deep Periphery). The Clans exist in a small islet of settled worlds in the Deep Periphery.