In 2398, a territorial dispute between the Capellan Confederation and the Free Worlds League erupted into a shooting war, as both ground and naval forces clashed in the Andurien system. This conflict was only the first of a dozen bloody but limited wars fought between 2398 and 2412 over ownership of key frontier wars, the first such wars in more than a century. A new era of bloody conflict had begun.

In 2412, after a fierce battle in the Tintavel system resulted in thousands of civilian casualties, representatives of the ten states of the Inner Sphere and the Periphery met in the city of New Olympia on the planet Ares to discuss a remarkable set of chivalric interstellar laws governing the conduct of war. Instead of attempting to prevent war, the Ares Conventions sought to legitimize its conduct, banning it in heavily populated areas and prohibiting military disruption of civilian economies. All six states of the Inner Sphere signed the agreement, but only two of the Periphery States did so.

As a result of the Ares Conventions, war became almost a continuous fact of life in the 25th century. It was transformed, however. From being an awesomely destructive event, war was now a curious, stylized feint and counterfeint in which outmaneuvered forces often surrendered rather than fight unfavorable odds. Compliance with the Conventions was almost universal, drastically reducing the human and economic costs of war. Unfortunately, it also promoted war as a means of resolving even the most minor dispute.

Throughout the next century and a half, the various interstellar states fought hundreds of little wars among themselves, all of them inconclusive. None of the governments was able to form either permanent, lasting alliances with one another to establish long-term supremacy over its neighbors. Nevertheless, the hereditary governments of these states survived the years of violence surprisingly intact.

The one exception came in 2459, when the childless, unmarried Lady Durant, leader of the Rim Worlds Republic, named Terens Amaris as her heir-apparent. In 2463, Lady Amaris succeeded Durant, and members of her line would rule in unbroken succession for almost three hundred years.

The Terran Hegemony had its fair share of battles during the Age of War. In 2431, Director-General Richard Cameron ordered his army to seize the Kentares system from the Federated Suns, and in 2475, it crushed a Free Worlds League invasion force at Oriente. In general, however, the leaders of the Hegemony hoped to avoid conflict, attempting to consolidate their power economically and technologically. Whatever the Hegemony's intentions, the state also became militarily superior with the invention of the BattleMech in 2439. Derived from the mining 'Mechs of the previous century and using the same myomer technology to power their movement, Terra's BattleMechs soon demonstrated greater mobility and adaptability to environments than conventional armored vehicles. They were also more heavily armed, with a full inventory of conventional and energy weaponry. The other states of the Inner Sphere also coveted 'Mech technology, but it only began to spread after a Lyran Commonwealth commando raid on the Hegemony 'Mech-production facility on Hesperus II in 2455. The Hegemony maintained superiority in the field, evolving new and better designs with more mobility and weaponry at lower cost and higher efficiency. Perhaps because of its military might, the Hegemony began to assume the role of mediator as the 25th century drew to a close.