A century and a half after the Age of War began, the Hegemony's role as mediator eventually brought an end to the wars through arbitration of a dispute between Houses Liao and Marik over the planet Andurien. In an irony of history, these same two combatants had fought over this world 150 years earlier, initiating the Age of War. Not only did Ian Cameron, 13th Director-General of the Hegemony. persuade the Capellan Confederation and Free Worlds League to sign peace agreements, but he earned the friendship of their leaders, Terrence Liao and Albert Marik. In 2556, the three states signed the Clasped Hands Agreement, a secret sub-treaty to the Andurien peace accords. The secret pact established special trade relations and promises of non-aggression. This led almost immediately to the Treaty of Geneva, which established these three as the founders of the Star League.

Between 2556 and 2569, Cameron used his mediator's skill to hammer out similar agreements with the Federated Suns, Lyran Commonwealth, and the Draconis Combine, the other three states within the borders of the Inner Sphere. In 2571, Cameron and the rulers of the other five states had established a new, unified hegemony called the Star League. In exchange for their recognition of Ian Cameron as First Lord of the Star League and arbiter of League foreign policy, the League Articles granted each of the other five leaders a seat on the High Council, autonomy over all domestic policy matters, and official sanction of the existing line of succession. Most important, all now had free access to the Terran military research apparatus.

The Periphery governments, on the other hand, wished to maintain their independence and resisted all diplomatic efforts to persuade them to join the League. These were the Outworlds Alliance, the Taurian Concordat, the Magistracy of Canopus, and the Rim Worlds Republic. Their traditions of autonomy were now centuries old, and they would fight to remain beyond the jurisdiction of the powerful Star League. The Periphery leaders began strengthening their militaries, knowing that confrontation was inevitable. In 2575, the Star League issued the Pollux Proclamation, ordering the Periphery states to join the League either voluntarily or by dint of force. Two months later, the Periphery states rejected the demand outright. In the next several years, the two sides fought a number of skirmishes and battles, but all-out war against the Periphery was not declared until 2578.

The Reunification War was a series of desperate campaigns that lasted 20 long years and claimed more soldier and civilian lives than the entire Age of War. It finally ended in 2596, following the final bloody campaign that led to the surrender of the Taurian Concordat. In 2597, the four conquered Periphery states became Territorial States of the Star League. The League immediately launched a vast public-relations campaign aimed at building popular support for membership among the people of the conquered territories. The effort was ultimately successful, with the League able to withdraw most of its garrison troops within ten years.