Most of Battletech history centers around five nation-states in a sphere around Terra (Earth) called The Inner Sphere:
Pictured: Instant status quo, just add twenty-five years and a few expendable NPCs.
The inhabitants of the Inner Sphere had beaten each other into a state of near barbarism, culminating in the fourth in a series of massive wars. In this last war, a sixth state was created to act as a buffer between two of the other states. This place was called The Free Rasalhague Republic, a(relatively) tiny collection of worlds created by political convenience:
Pictured: Added NPCs.
In 3050, a coalition of long-lost colonies arrived in the Inner Sphere, determined to take over everything. They called themselves The Clans, and they had spent all of the years that The Inner Sphere had used grinding their best technology into dust to make deadly machines of war piloted by 'Trueborn' genetic super-soldiers.
Sadly, the nacent Free Rasalhague Republic was in their path. Millions were killed, and millions more were taken as 'Freeborn' fen to add to the citizenry of their new gene-tweaked overlords. Some Inner Sphere warriors, including those of the FRR army (or Kungsarme) were taken into Clans as Bondsmen; honored slaves just a cut above the masses.
Important turning points of the war were Luthien, the capitol of a neighboring state (the Draconis Combine) and Tukayyid, an unassuming agrarial world in the Free Rasalhague Republic. Tukayyid marked a treaty point in The Clan Invasion where the Inner Sphere manipulated the Clan honor code, which focused on bravado and dealmaking to prove a warrior's mettle. A cease fire of fifteen years was granted by the Clans.
Pictured, in order from Top to Bottom: Fearsome Invaders, Scrappy Underdogs
Lyon's Thumb is a cluster of systems with habitable planets found in the Free Rasalhague's other neighbor, the Federated Commonwealth, led by the Steiner-Davion royal family.
Outreach(not pictured) is the home of most mercenary business in The Inner Sphere. It's overseen by the most reknown mercenary unit, The Wolf's Dragoons. The Wolves perform accredation of mercenary units, offer repair and training services, serve as hiring brokers, gather relevant intelligence on such units, and hold mercenary units accountable for the comission of war crimes, breaches of contract, or other infractions which might reflect poorly on the mercenary business as a whole. The mercenary industry is usually booming in the Inner Sphere, as large governments with slow methods of interstellar travel and relatively small armies often need professional military forces in the right place at the right time. The Clans do not hire mercenaries and their culture holds them in the very lowest of regards.
During 3058, the Jade Falcons, within the confines set by the Treaty of Tukayyid, launched a surprise assault on several worlds in the Federated Commonweath, cutting a swath through the FC that terminated on Coventry. It was a show of force that left their other posessions in the Inner Sphere poorly defended by reserves and salhoma troops, those deemed too old to be effective warriors.
Comstar was an interstellar communications agency made into an independent organization so that it could serve a neutral arbiter between Successor States and allow communications between planets to continue even through a series of destructive wars. This neutrality allowed Comstar to maintain quite a bit of technology that the Successor States did not, and it did so rather quietly for a variety of reasons. Comstar forces were the ones who negotiated, fought, and won the Treaty at Tukayyid.
When Comstar later liberalized and began sharing thier technology in an attempt to help the rest of The Inner Sphere fight The Clans, factions within it which had turned their secrecy into a rote religion separated from Comstar and named themselves after the organization's creator, Jerome Blake. The Word of Blake would later use Comstar secrets to engage in all manner of fanatical, political acts over the next fifteen years before radically redefining baselines for 'fanatical,' 'political,' and 'acts.'
Around 3060, amongst a handful of political criseses that cast doubt on the firmness of The Treaty of Tukayyid, the Inner Sphere did decide to go on the offensive against a Clan. They officially united under the banner of the then-and-now-defunct Star League and chose a target: The Smoke Jaguars. Operation Bulldog was put into effect, slamming waves of elite, Inner Sphere troops into the unprepared Clan forces. The Operation was an unexpected success, and The Star League Defense Forces resolved to follow the Smoke Jaguar forces all the way back to their holdings in The Clan Homeworlds.
Pictured: Yes, the five bigger blobs annihilating the smaller blue blob (not pictured) was a really big deal. Somehow, the dead guys are considered the jerks in this scenario.
A few years later, Clan Ghost Bear, now sharing a border with The Draconis Combine in the wake of the Jaguars' removal, became embroiled in a war. While the war was started by rogue forces, both sides were drawn in by codes of honor that could only be satisifed with more bloodshed. The conflict ended in a quick, but very tenuous peace that allowed both parties to save face. Raids by rogue units jeopardized this peace and both sides worked fervently to quell their own problems before another costly war could start.
Also see the Battletech Mini-Glossary.
Pictures courtesy of Sarna.com.