Lasting from 3028 to 3030, the Fourth Succession War was the largest conflict since the Second Succession War, over a hundred years prior. The war gave the Lyran Commonwealth and Federated Suns a common border so that they could eventually have one contiguous nation, the Federated Commonwealth.
The planning for the Fourth War came after Federated Suns First Prince Hanse Davion signed the Federated Commonwealth Alliance document with the Lyran Commonwealth Archon, Katrina Steiner, in 3022. 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 began by staging military exercises close to the Capellan Confederation border on a yearly basis starting in 3026. The so-called "Galahad tournaments" not only gave his troops exercise, but it got the Capellans used to large troop movements near their border.
On the Lyran side, Archon Katrina was at first extremely reluctant about a joint offensive with the Federated Suns. However, intelligence reports indicated growing increased public unrest in the Rasalhague military district. Since the district's governor, Mies Kurita, exaggerated the insurrection for several years to get his way, even the frightening increase of hostilities against the Draconis Combine was ignored by the Combine's government and even by DCMS commanders within the district.
The LIC made good use of the situation by supporting the Rasalhaguian resistance, the so-called Tyr Resistance Movement. 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 addition, LIC-analysts believed, that the DCMS-units along the border made a critical error: they felt into garrison routines. Retirees, recruits and supplies were provided according to a rigorous timetable, resulting in a very seasonal strength of individual units along the border with the Lyran Commonwealth.
Considering the situation along the border, Archon Katrina Steiner agreed with Prince Hanse Davion's plan. 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 Maximilian Liao were informed that the other party would be involved. The Accords were signed, but did not overcome long years of distrust.
The Federated Suns conquered roughly a third of the Capellan Confederation and two commonalities seceded from the realm. The CCAF lost nearly two-thirds of their combat strength. In addition, the AFFS captured the bulk of the Capellan heavy industries, including the Earthworks factory on Tikonov. While the Tikonov Free Republic joined the Federated Suns, the St. Ives Compact continued to remain politically independent, though dependent on AFFS military aid to avoid being conquered by the Capellan Confederation. The conquered worlds were incorporated into the Federated Commonwealth as the Sarna March.
Maximilian 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 Suns that included stationing ComGuard troops within FedSuns space to protect their HPGs from further attack. Secretly, ComStar's intelligence agency, ROM, and MIIO fought a shadow war for years.
The Lyran Commonwealth captured a stunning 53 planets from the Draconis Combine, many of which were former Commonwealth worlds. The Lyrans gained additional planets from the Free Worlds League. Since the Lyrans didn't 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. Thus, the Lyran Commonwealth had no choice but to cede their conquered worlds to the new Free Rasalhague Republic.
The Draconis Combine lost 53 worlds along the Lyran border, mostly in the Rasalhaague and Dieron Military Districts and captured in return only two Lyran planets. However, the DCMS captured a few planets from the Federated Suns, but the gains were nowhere near the Combine's losses. In addition, many of its best regiment were destroyed or severely damaged by either the LCAF or Wolf's Dragoons. The only bright spot for the Combine was the rise of Theodore Kurita during the war.