After the fall of the Star League, the planets of the Terran Hegemony were divided up between the Great Houses with the exception of Terra, which became a neutral ground controlled by ComStar. The worlds of what would someday become the Chaos March were mostly claimed by the Capellan Confederation, but a few fell under the sway of the Free Worlds League. Over the years, however, four violent wars wracked the Inner Sphere, which became known as the Succession Wars. The Fourth Succession War resulted in the transfer of many worlds formerly belonging to the Capellan Confederation and Free Worlds League to the control of the nascent Federated Commonwealth. These worlds were organized within the Federated Commonwealth as the Sarna March.
In 3057, Chancellor Sun-Tzu Liao and Captain-General Thomas Marik launched a strike at the Federated Commonwealth in retribution for Victor Steiner-Davion's Operation Doppelganger, which was a plot to replace Marik's son with a body-double. This so-called Operation Guerrero was stalled when Katherine Steiner-Davion seceded the Lyran half of the Federated Commonwealth and negotiated a separate settlement with Marik that allowed him to achieve his objective. Katherine ordered home all traditional Lyran units to within the Lyran state's pre-Fourth War borders, leaving many worlds bereft of defenses. Because of the secession, the rump Commonwealth lacked the JumpShips necessary to reinforce the Sarna March. With the League's pull-out, however, the Capellans lacked the military force to secure the worlds that the Commonwealth had all but abandoned.
Prior to the offensive, Capellan terrorists had stirred up old Capellan loyalties amongst the populace, as well as masterminding many terrorist attacks targeting the infrastructure of these systems. In the absence of national militaries, many local governors (and other concerned interests) seized the initiative to restore order. On the worlds were this restoration was successful, most governments were resentful of the apparent lack of interest by the Great Houses, and set up their own nations. Some of these minor powers became influential enough to become multi-world nation-states. The constantly shifting web of alliances, as well as the rapid rise and fall of planetary-level governments led observers to dub this area of instability the Chaos March.
Over time, both the Capellan Confederation and the Federated Commonwealth were able to exert more influence and military pressure on the worlds of the Chaos March, leading to its shrinkage. Though both would become distracted (the Capellans by the war with St. Ives and the Commonwealth by the Clans), many worlds of the Chaos March had been reclaimed by one of the Great Houses.
When the FedCom Civil War broke out in 3063, some Chaos March worlds were caught in the middle. Indeed, Duke George Hasek was tired of the Capellans constantly bullying the small nations, so he undertook a campaign to "liberate" some of them.
The face of the Chaos March changed forever when the owners of Terra, the Word of Blake announced the formation of the Blake Protectorate in 3066. The Protecterate was composed of several former Chaos March worlds who were bribed or otherwise coerced into joining the Blakist state. The Word of Blake Jihad that started in 3068 forced several other worlds into the Protectorate, including some that had rejoined the recently-renamed Federated Suns.
The Jihad engulfed the Inner Sphere until Devlin Stone was able to organize an effective military force to defeat them. Stone then claimed many planets near Terra as part of his new nation, the Republic of the Sphere. All of the worlds of the former Chaos March became a part of the Republic. In 3130, the Chaos March is but a memory.