The Third Succession War ran from 2866 to approximately 3025. Compared to the previous Succession Wars, the Third was marked with almost constant, but low-intensity, warfare as the Successor States fought over increasingly dwindling resources and technology.
After three decades of high-intensity warfare, the Second Succession War spluttered to a conclusion as fighting tapered off in 2863. As with the similar conclusion of the First Succession War, with the conflict ending due to both military and economic exhaustion of all parties rather than any decisive victory, tensions between the Successor States remained high. Despite this, many hoped peace would prevail as the Inner Sphere went nearly two years without conflict, with Lyran Commonwealth's Estates-General and Free Worlds League Parliament both pressing for stand-down of their respective militaries, a refocusing on civilian reconstruction and end to war-time austerity measures.
In response to efforts by these parties, Archon Elizabeth Steiner attempted to broker a formal peace treaty with Captain-General Philippa Marik in 2864, the success of these prompting the Captain-General to request these efforts be expanded to include the other Successor States. Representatives from all five houses met on New Earth in November 2864, but it soon became clear just how divided the Inner Sphere remained. The Draconis Combine representative indicated he was nothing more than an observer and that any treaty the others agreed on would be approved or rejected by Luthien at a future date, the Capellan Confederation refused to sign any treaty that did not make provision for the return of all the territories it had lost in last two wars, which the Free Worlds League and Federated Suns vociferously opposed, with the FedSuns representative Luther Davion making equally unrealistic claims for the other houses to accept him for position of First Lord. Aside from the Combine representative being arrested and executed by the Lyrans as a spy, the talks collapsed after seven fruitless weeks with nothing being achieved.
A group of Capellan and Free Worlds businessmen under the banner of the "Peace of Money Movement" likewise attempted push for a permanent peace treaty, but aside from First Prince Michael Davion, none of the other Successor Lords would give them the time of day, with a number of the other Houses convinced that First Prince Michael's enthusiastic and lavish support meant the movement was actually a Davion plot and the movement's claims of the Lyrans profiteering from the previous conflict alienating the Commonwealth.
While many in the Inner Sphere hoped peace had finally come even if the formal attempts at peace treaties had failed, the bitter animosities between the Successor States meant conflict could just as easily resume, needing only to be triggered by those who wanted the war resumed, such as the quasi-religious communications company ComStar. Focused on the supposed "Word of Blake", ComStar guarded and preserved the Star League's technological secrets in the belief that when the Inner Sphere collapsed after the Great Houses had destroyed themselves, ComStar would rise to power as the savior of humanity. Seeing the prophecy foretold by the Blessed Blake endangered when Precentor ROM provided evidence that Inner Sphere would likely recover the bulk of Star League LosTech within three decades, in 2866 Primus Raymond Karpov and the First Circuit authorized ROM to filter information about the so-called Shadow War in the Draconis Combine between elements of the Internal Security Force loyal to Coordinator Miyogi Kurita and his sister Roweena to agents of Lyran Commonwealth and Federated Suns. Just as Karpov had hoped, in typical Kurita fashion when Miyogi learned that outsiders knew of the Combine's internal troubles and with Lyran traders making embarrassing inroads in the Rasalhague Military District, the simmering tensions along the Combine-Commonwealth border exploded into open warfare again on the Commonwealth's coreward border that soon swept across the entire Inner Sphere, signaling the beginning of the Third Succession War.
The intensity of this conflict never approached that of either the First or Second Succession Wars. This lessening was more likely due to reduced military capacity rather than a change of tactics. The scavenger necessities that had begun to emerge during the Second Succession War carried over and each Great House rapidly realized that they needed to preserve their precious technical resources.
Two main components of this era were the evolution of a set of informal rules of engagement, similar to old Terran chivalric values, and a neo-feudal system. The rules of engagement were similar to the Ares Conventions whereby 'Mech forces fought over industrial facilities, but were restricted in their attacks on those facilities and civilians. Major battles, especially between mercenary units, were fought in stages with both sides allowing enemy Techs to enter the battlefield during periods of truce to attend to damaged 'Mechs. Especially important was a tradition of paying a ransom for safe passage after surrendering to a superior force. Also, JumpShips became strictly off-limits as a military target as it was quickly recognized that without JumpShips, nothing would be possible and interstellar civilization itself would collapse.
The feudal system evolved throughout the years because, just as during Terra's original Dark Ages, the capacity for central governance of the Inner Sphere's stellar nations had evaporated. As a result, each Successor Lord controlled their house via a hierarchy of nobles who, in exchange for full authority over their worlds, pledged their fealty and their 'Mechs to the service of their liege lord.
The Third Succession War drew to a close ironically not through any military victory but with another call for lasting peace. Recognizing humanity's slow but steady slide away from the glories of the Star League-era and toward barbarism, Archon Katrina Steiner of the Lyran Commonwealth issued a Peace Proposal to the other House leaders in 3020. Hanse Davion, First Prince of the Federated Suns, was the only Successor Lord who welcomed the initiative. The result of their talks was the Federated Commonwealth Alliance Document, which Prince Hanse Davion and Archon Katrina Steiner signed on Terra in 3022. In addition to trade and military agreements, the F-C Document contained a secret provision betrothing the Archon's daughter in marriage to Hanse Davion. Though the marriage would not occur until the twelve-year-old Melissa came of age, this was the first step on the road to eventual unification of the Lyran Commonwealth and the Federated Suns.
The other three Successor Lords were aghast at this sudden shift of power, which would lead to the eventual creation of a single state nearly as large as their three Houses combined. In October 3022, the Draconis Combine, the Free Worlds League, and the Capellan Confederation hastily signed the Concord of Kapteyn. Though not as far-reaching in scope as the F-C Document, it called for an end to active hostilities between the three states, who now pledge mutual defense against combined Federated Commonwealth as well as agreements to coordinate their overt and covert efforts to derail the Davion-Steiner alliance.
Perhaps most active in the covert attempts was the Capellan Confederation under the leadership of Chancellor Maximilian Liao. Liao agents had already subverted Hanse Davion's brother-in-law Michael Hasek-Davion, who was now providing them with valuable military intelligence. When the Chancellor next initiated Operation Doppelganger, a scheme to kidnap and kill Hanse Davion in order to replace him with a doppelgänger his scientists had created, he went too far. Davion eventually escaped capture and imprisonment and returned to his place on the throne, vowing revenge and setting the scene for the Fourth Succession War.