History of the Succession Wars
The Succession Wars were one of the most devastating conflicts ever, engulfing the Inner Sphere in nearly constant fighting for three hundred years following the fall of the Star League. With the destruction of House Cameron, the five remaining Great Houses almost immediately started fighting each other over the position of First Lord. The devastation was unimaginable: untold numbers of casualties, hundreds of worlds rendered lifeless, and technological progress pushed back centuries as much of the knowledge humanity had gained in the preceding years became lostech. 
In the wake of the Star League Civil War, the High Council reconvened in the ruins of Unity City on 10 October 2780. The bickering began almost immediately as, with the death of the Cameron line, the most pressing matter became determining who would assume the position of First Lord. One of the few things the Council Lords could agree on was the appointment of Jerome Blake as Minister of Communications to restore the League's communications network. The other was that Aleksandr Kerensky, hero of the Civil War, would not become the next First Lord. On 18 October they formally stripped General Kerensky of his title as Protector of the Star League.
For the next ten months the Council Lords attempted to convince their peers to elect them as the new First Lord; entire planets were up for sale as they haggled with each other to gain support for their claim. Realization eventually settled in that there could be no agreement on the matter, and finally a short statement was given to the media: "After long months of intense negotiations, we have reached an impasse on the question of who should become the next leader of the Star League. It is our opinion that the inability to find a new First Lord makes any further decision-making impossible. Therefore, we officially dissolve the High Council on this day, August 12, in the year 2781".
Although there was popular support for his seizing power, General Kerensky was determined to save the Star League, and for two years he shuttled between the leaders of the Great Houses in an attempt to get them to reconcile. The House Lords were unmoved though and began to massively build up their militaries, recruiting even among Rim Worlds Republic prisoners. When the recruiters began soliciting Star League Defense Force units, General Kerensky decided there was only one way to save the spirit of the Star League. His plan, Operation Exodus, saw himself and the vast bulk of the SLDF disappear from the Inner Sphere on 5 November 2784, never to be seen again. In the power vacuum which followed, the Great Houses began to seize worlds of the Terran Hegemony for themselves.
First Succession War
When Coordinator Minoru Kurita unilaterally declared himself the new First Lord in 2786, the other House Lords quickly followed suit, touching off the First Succession War. The Successor States invaded each other with their massive armies while Jerome Blake seized Terra himself to serve as the headquarters for the newly-christened ComStar. The war would last for 35 years and become the single bloodiest conflict in human history: ignoring the Ares Conventions, all sides made use of weapons of mass destruction, wiping out entire cities and rendering whole planets uninhabitable.
When the war finally ground to a halt in 2821, it was more out of sheer exhaustion than anything else. None of the Successor States were any closer to their original goal, but the scale of the atrocities which had been committed meant none were willing to make a lasting peace either. The next few years were dedicated to rebuilding, but with their crippled infrastructures and the deaths of so many scientists and technicians the task was immense. Nine out of every ten WarShips and nearly half of all JumpShips had been destroyed during the war, but with the heavy targeting of the shipbuilding industry the ability to replace these losses was virtually non-existent.
Second Succession War
The Second Succession War began in 2830 when Jinjiro Kurita launched all-out offensives against the neighboring realms, provoked in part by ComStar secretly feeding information to the Lyran Commonwealth. No less bloody than the first, the Second Succession War also ended inconclusively, grinding to a halt in 2864. The technological decline brought about by the first war continued unabated: many worlds regressed to the level of 20th century Terra, if not further. Even for the few worlds which were able to stave off such a drastic regression, none could boast technology more advanced than what was available at the start of the Age of War. This decline was helped in no small part by ComStar itself, whose Operation Holy Shroud actively sought to destroy any knowledge which it didn't monopolize.
Their ability to reproduce advanced computers, large fusion engines and other technological devices severely hampered, the Successor States took to cannibalizing existing equipment just to keep their current inventory of war machines operational. The last WarShips were lost in battle during the war, rendering the class extinct in the Inner Sphere. With a similar fate awaiting the few remaining JumpShips (and with them all hope of interstellar travel), an unwritten agreement was formed whereby these ships were now off-limits in battle. Thanks to the Communications Protocol of 2787, ComStar was able to preserve the Hyperpulse Generator network, though under Blake's successor Conrad Toyama their operation began to take on a quasi-mystical nature.
Third Succession War
The Third Succession War began similarly to the second, with ComStar secretly supplying information to the Lyrans, thereby provoking Miyogi Kurita into attacking them. Lasting from 2866 to 3025, the war was characterized by its low-tensity nature and the rise of neo-feudalistic traditions in all of the Successor States.
An informal set of rules took hold as the Successor States realized the few remaining vital resources had to be protected. With JumpShips too precious to destroy, ground combat began to play a more prominent role, with aerospace forces taking on an increasingly supporting nature. Vital objectives like factories and cities were still fought over, but very rarely within them so as to avoid collateral damage. Battles, particularly between mercenary units, were conducted in stages to allow Techs to scavenge the battlefields while MechWarriors were more willing to surrender to a superior force so they could pay their ransom and escape off-world. Meanwhile, the destruction from the two previous wars had made it more difficult for the Great House families to exert centralized control over their realms, forcing them to rely more and more on the nobility class to govern their state.
Towards the end of the war, and despite ComStar's best efforts, the Successor States had made small gains in recovering some of what had been lost over the last few centuries. However, the greatest development came in 3028 with the discovery of the Helm Memory Core. The unveiling of an intact Star League database brought with it the promise that humanity would be able to claw its way out of the darkness of the Succession Wars.
Fourth Succession War
Lasting from 3028 to 3030 the Fourth Succession War was a short and brutal war between two emerging power blocs. Declared during the marriage of Hanse Davion and Melissa Steiner, the Fourth Succession War was a conflict started by the newly formed Federated Commonwealth against the ComStar-sponsored Concord of Kapteyn.
One of the few conflicts that had a clear winner, the Fourth Succession War saw the the emergence of the new Federated Commonwealth as the most powerful nation since the original Star League. The Capellan Confederation's territory and power was cut in half, with many of its former worlds under the control of the new FedCom alliance or forming part of the Tikonov Free Republic and St. Ives Compact. The Draconis Combine also lost a large number of worlds to Steiner forces, though it managed to capture a few Davion planets in recompense.
The Federated Commonwealth dominated the Inner Sphere like no power before the start of the Succession Wars, but its creation did not lead to the end of the fighting. Seeking to create a buffer between it and the new super-state, the Combine allowed for the formation of the Free Rasalhague Republic, an action which sparked the so-called Ronin War. The Duchy of Andurien, hoping to take advantage of a mortal enemy in its weakened state, formally seceded from the Free Worlds League and launched an invasion of the Confederation; the Andurien Crisis finally ended with Andurien being forcibly returned to the League.
Hoping to knock out the Combine as they had the Confederation, the Federated Commonwealth launched the War of 3039 but were forced to back off when the Kuritans (with ComStar assistance) proved more formidable than expected. Still, the expectation was that a new Succession War would begin as soon as the Federated Commonwealth sought to unite the Inner Sphere under one rule once more...until 3049 when the Clans, the descendents of the SLDF exiles, launched their own invasion, touching off a new era in history.