The Fourth Succession War occurred in the fictional BattleTech universe.
Planning for war
The planning for the Fourth War came after Federated Suns Prince Hanse Davion signed the Federated Commonwealth Alliance document with the Lyran Commonwealth Archon, Katrina Steiner, in the late 3020s. Secretly included in the document was a provision that called for the marriage of Hanse and Archon-designate Melissa Steiner, which would (eventually) result in the union of those two realms in the person of Hanse and Melissa's eldest child. In order to join their realms together, they needed to create a common border. This could only be done through the conquest of intervening planets.
Hanse Davion also had a score to settle. A few years prior, Chancellor Maximillian Liao had attempted to replace Prince Davion with a body-double. This double certainly would have delivered the Federated Suns into the Capellan Confederation's hands, but the plot was discovered and foiled by Hanse's best friend and closest advisor, Ardan Sortek.
Hanse began by staging military exercises close to the Capellan Confederation border on a yearly basis. The so-called Galahad tournaments not only gave his troops exercise, but it got the Capellans used to large troop movements near their border.
Hanse had also instituted an elaborate spy game. The son of the director of the Federated Suns civilian spy agency (the Ministry of Information, Intelligence, and Operations or MIIO for short), Quintus Allard, Justin Allard was publicy ridiculed and exiled. After he publicly embraced his Capellan heritage, started calling himself Justin Xiang (his mother's maiden name), and won the Solaris VII championship, Chancellor Liao welcomed him into his intelligence agency. Unbenownst to all but Justin, Hanse, and Quintus; Justin Xiang Allard was actually a double agent working for his father. In addition, another deep cover agent, Alex Mallory, had also infiltrated the Maskirovka (the Capellan spy agency). By the time the war started, these two men were the #2 and #3 men in the Maskirovka.
Hanse was also aware of a traitor in his midst. His half-stepbrother, Duke Michael Hasek-Davion, nursed a bitter grudge against Hanse because he wanted to be Prince (for a more detailed analysis, see this article). In short, Hanse used Michael to feed false troop information to the Capellans, who became woefully out of position for the assault.
Prince Davion had also signed an independent truce with Candace Liao, heir to the Chancellorship and Duchess of the St. Ives Commonality (roughly one-fifth of the Capellan Confederation).
On the Lyran side, Archon Katrina made contacts amongst the Rasalhaguian resistance. The conquered people of Rasalhague had maintained an organized resistance for centuries, even though the majority of it had been stopped by the mid-24th century. The Lyrans used this resistance in exchange for the promise of freedom.
In response to the F-C alliance, the leaders of the other three Successor States, the Capellan Confederation, Free Worlds League, and Draconis Combine came together under a loose alliance known as the Kapteyn Accords. This alliance, which was overseen by ComStar, lacked the uniting features of the FedCom alliance. Indeed, negotiations were almost derailed when Janos Marik and Maximillian Liao were informed that the other party would be involved. The Accords were signed, but did not overcome long years of distrust.
Uniting Two Families
Perhaps the most important social event until that time occurred on Terra. Hanse Davion married Melissa Steiner. The leader of every Successor State was there, including their aides, heirs, and other VIPs. At the wedding reception, Hanse Davion uttered one of the most quoted lines in Inner Sphere history, "My dear... I give you the Capellan Confederation." To the leaders of the Inner Sphere, it seemed like this touched off the 4th War, but, in fact, troops had already invaded the day before.
The Capellan Front
The most damage during the war was done to the Capellan Confederation. Davion's stated goal was to ensure that the Capellan Confederation was no longer a threat by depriving it of its heavy industry. Under the cover of war games, the Armed Forces of the Federated Suns (AFFS) used sheer numbers to overwhelm the Capellan Confederation Armed Forces (CCAF).
The AFFS exploited a strategic weakness in CCAF doctrine. The Capellans lightly garrisoned worlds and relied on a strategic reserve to intervene quickly to achieve local superiority. This was well-suited to dealing with the constant raiding of the Third Succession War, but left the Capellans unable to respond when multiple planets were hit at the same time. Coupled with overwhemling force and a better-trained AFFS, the Capellan all but folded.
Chancellor Liao also had to deal with a high-ranking traitor. Pavel Ridzik, Maximillian's chief military advisor decided to seize power for himself. The Tikonov native returned to his homeworld and announced that he was forming his own nation, the Tikonov Free Republic. The Republic signed an independent alliance with House Davion and attacked the Free Worlds League. In retribution, Chancellor Liao had Ridzik assassinated. Ardan Sortek, who had been in the Repulic as an advisor, took temporary control of the Republic. After the end of the war, the Tikonov Free Republic voted to join the Federated Suns around 3040.
The truce that Hanse Davion had struck with Candace Liao paid off. Amid a romance with Candace, Justin Xiang Allard outed himself as a Federated Suns agent to her and returned the Federated Suns space with her. She brought the St. Ives Commonality with her and formed her own independent nation, the St. Ives Compact. This decision left her younger sister, Romano as the heir-apparent to the Celestial Throne.
ComStar also took an interest in the course of the war. The supposedly neutral power desired smaller, more fragmented nation that would be easier to manipulate, but the 4th War seemed to promise a mighty Federated Commonwealth. ComStar decided to slow the AFFS' progress by imposing a communications interdiction. They faked an AFFS attack on the HPG on Sarna and used the "incident" as the excuse they needed. Unbeknownst to ComStar, Hanse Davion had changed the units he used to assault Sarna at the last minute, alerting him to ComStar's duplicity. Despite that, Hanse could not afford to openly challenge ComStar, so he made no public statement about it. The interdiction effectively stopped all interplanetary communication within the Federated Suns, as well as any communication in or out. Hanse still had another trick up his sleeve. The New Avalon Institute of Science had recently duplicated the ability to send communications between worlds with their fax machines, but they were much slower than what ComStar could manage. These so-called "black boxes" were pressed into service, but Hanse found it increasingly difficult to coordinate his military.
ComStar showed their perfidy once more towards the end of the war. In order to set back the technological progress of the Successor States, ComStar executed a raid on the New Avalon Institute of Science. The disguised themselves as the Death Commandos, the elite commandos of the Capellan Confederation and personal bodyguards of Chancellor Liao. During the battle, Prince Hanse Davion took to the field himself in his BattleMaster. The attack did only superficial damage to the university, but did great damage to ComStar in Hanse Davion's eyes. His spies in the Capellan Confederation later reported to him that the Death Commandos were on the planet Kathil during the timed of the raid, so there was no way they could have pulled it off. Coupled with the knowledge of the supposed HPG destruction on Sarna, that knowledge led Hanse to the only logical conclusion.
The Combine Front
In order to find the forces for his massive assault on the Capellan Confederation, Hanse Davion had to strip away large quantities of troops from the border with the Draconis Combine. Far from leaving himself open to attack, Hanse contracted the mercenary unit Wolf's Dragoons to defend his border. Jaime Wolf, the commander of the Dragoons, and Coordinator Takashi Kurita held a deep personal dislike of each other, so Jaime was able to goad Coordinator Kurita into throwing everything he could spare at his Dragoons.
By the end of the war, Wolf's Dragoons had been reduced from over five regiments to one reinforced regiment. In gratitude, Hanse Davion granted Jaime Wolf and his Dragoons the conquered planet of Outreach in perpetuity.
The Combine Front
The Lyrans had many things going for them. They had the Rasalhague rebels fomenting insurrection and providing them with intelligence. They had Takashi Kurita's distraction with Wolf's Dragoons. They used these advantages to push the Draconis Combine back along a very broad front. They took a number of traditionally Rasalhagian worlds from the Combine, though their successes did not match what the Federated Suns managed to achieve against the Capellans.
Lyran military doctrine was significantly different than that of the Armed Forces of the Federated Suns (AFFS). Whereas the AFFS executed several different "waves", effectively leapfrogging each other, the Lyrans used their wall of assault 'Mechs and pushed into the Draconis Combine on a very broad front. This was not a tactic that had served the Lyrans well in the past, circumstances conspired to make it a feasible strategy for the 4th War.
The Lyran offensive was stalled when the Combine struck back and scored a couple of worlds that put them within one jump of Skye, a regional capital. Unwilling to continue pressing forward, but unable to reclaim their lost worlds, the Lyran assault ground to a halt.
The League Front
Rather than face the onslaught of the Lyran military, Captain-General Janos Marik made the decision the execute a "strategic withdrawal" of forces along the Lyran front, essentially handing over dozens of worlds without a shot fired. Focused on the Draconis Combine, Archon Katrina made no moves to penetrate any further into the Free Worlds League.
Free Worlds League
Meanwhile, both Maximillian Liao and Hanse Davion had been sponsoring a rebellion in the Free Worlds League. Anton Marik, in an ill-advised revolt, attempted to wrest power away from his father, Captain-General Janos Marik. The anti-climatic end of battle occurred when he turned on the mercenaries that Liao had provided him, so Natasha Kerensky's Black Widows killed Anton. With the Free Worlds League Military freed up, Janos decided to take advantage of Capellan distraction and attack. Hanse, however, did not want the Free Worlds League to strengthen, so he declared an end to the war.
The Federated Suns conquered roughly a third of the Capellan Confederation, while two commonalities seceded from the realm. While the Tikonov Free Republic joined the Federated Suns, the St. Ives Compact continued to remain politically independent, though dependent on Armed Forces of the Federated Suns military aid to avoid being conquered by the Capellan Confederation. These conquered worlds were incorporated into the Federated Commonwealth as the Sarna March.
Maximillian Liao went stark-raving mad. His younger daughter, Romano Liao ran the nation until his suicide. With her military shattered, and much of her nation's industry under foreign control, Romano struggled to ensure the Capellan Confederation's survival. It would be decades before the Capellan Confederation would be a threat once more.
ComStar and the Federated Suns negotiated a settlement to restore communications to the Federated Suns that included stationing ComGuard troops within Federated Suns space to protect their HPGs from further attack. Secretly, ComStar's intelligence agency, ROM, and MIIO fought a shadow war for years.
When the Lyrans failed to retake every Rasalhague world, they declared that they could not give the Rasalhagians their independence. Not content with that, the Rasalhagians grumbled until ComStar negotiated a settlement with the Draconis Combine that allowed for Rasalhagian independence. Shamed by the Draconis Combine's willingness to grant the Rasalhagians independence, the Lyran Commonwealth had no choice but to cede their conquered worlds to the new Free Rasalhague Republic.