The Ares Conventions were a treaty signed in New Olympia on the planet Ares during the Age of War, aiming to reduce the loss of civilian life by establishing a code of conduct during warfare. The effects of the Ares Conventions were mixed. On the one hand compliance was almost universal and reduced both the human and economic costs of war. Unfortunately, the Conventions also had the effect of legitimizing warfare as a means of resolving even the slightest disputes. The end result was a century and a half of constant but low-level fighting between all of the major powers.
The Conventions were upheld until the Reunification War when the Star League chose to wage total war against the resistant Periphery states, and were formally renounced at the beginning of the First Succession War. Despite being considered no longer binding, the Ares Conventions continue to be seen as the guide for civilized warfare.
In 2412, the forces of Captain-General Peter Marik invaded the Capellan world of Tintavel. While at first a traditional assault, the defenders resorted to hit-and-run tactics and hiding in cities. After the fighting had already badly damaged the planetary infrastructure and tens of thousands of civilians had been killed in urban warfare, both commanders resorted to using strategic-level weapons of mass destruction. In the end, both Captain-General Marik and Capellan Chancellor Aleisha Liao ordered their forces to withdraw, and within a few years the planet was completely abandoned.
The carnage so appalled Aleisha Liao that she began a campaign of aggressive diplomacy, succeeding in establishing a conclave of diplomats and House leaders - the Ares Conference, which was tasked with creating rules of war which would reduce civilian losses. While the Conference was initially in favor of Aleisha's proposal, the face-to-face meeting between the national leaders revived simmering rivalries and general mistrust. Starting with 80 pages of the original draft, the Conventions bloated up as more and more clarifications were added to prevent a future "misunderstanding" during wartime.
In the end, the Ares Conference drew up a massive treaty consisting of 6 articles and 18 appendices, totaling 320 pages. The Ares Conventions were signed on the 13th of June, 2412 by eight of the ten nations invited: the Terran Hegemony, Capellan Confederation, Free Worlds League, Rim Worlds Republic, Federated Suns, Principality of Rasalhague, Lyran Commonwealth, and Draconis Combine.
While even some of the signatories were critical towards the Conventions, the Taurian Concordat and the United Hindu Collective both abstained entirely. The Concordat was distrustful of the Capellans in general and considered the Conventions hypocritical; the Taurians were fighting a border conflict with the Confederation, during which Liao forces had conducted massacres similar in scale to Tintavel before, during, and after the Conference. The Collective politely declined as they had no interest in waging any wars, and feared that the Conventions would legitimize and trivialize warfare.
- The Preamble announced the signing of the Conventions on June 13, 2412.
- Article I forbade the use of nuclear weapons against all civilian targets and planets and military targets within 75,000 kilometers of a planet.
- Article II forbade orbital bombardment except against vital military targets which were not anywhere near populated areas.
- Article III established the white flag with adorned red S as the universal symbol of surrender and truce, which all signatories agreed to abide by.
- Article IV established the right of safe passage under a sign of truce and conditions for its loss.
- Article V explicitly disavowed combat in cities - unless a military target was within the city - and against civilian targets.
- Article VI forbade research, development, and use of biological and chemical weapons.
- Appendix A comprised of the definition of combat forces, ranging from definitions of infantry to aerospace forces. Since the Conventions were supposed to be followed by uniformed combatants, extensive definitions of uniforms were also part of this section.
- Appendix B defined what a valid military target during warfare was.
- Appendices C and D defined civilians and civilian assets. Notable for their rigidity, these appendices created the modern definition of a pirate merely by omission, as a pirate was neither military personnel nor a civilian.
- Appendix E clarified the rules for surrender and treatment of surrendered forces, as well as safe passage for humanitarians, noncombatants, and civilians through hostile territory.
- Appendices F through H defined the nature of military force and hostile action. Aside from seeking to limit open conflicts, these appendices introduced the idea of conducting proxy battles via sports matches, simulation games, or duels.
- Appendices I through L defined weapons of mass destruction. The restrictions for their use were so strict that not even tear gas could be used against foreign troops.
- Appendices M through O defined boards of inquiry and investigative commissions for violations of the Ares Conventions. In practice, the neutral party required for investigations turned out to be the Terran Hegemony, which would eventually use its diplomatic influence and prestige to gain support for the Star League.
- Appendices P and Q formed the Conventions' glossaries. These two appendices were filled with both definitions of common terms used thorough the Conventions, as well as direct translations into languages other than English. Notably for its miserable Hindi translation, which rendered large sections of the translated text incomprehensible.
- Appendix R defined the allowable uses of espionage and intelligence operations. Unlike the rest of the Conventions, this appendix was far less restrictive and only banned outright assassination.
- Addendum I was first proposed in 2465 by the Terran Hegemony. It added definitions for BattleMechs to Appendix I. This Addendum was last signed in 2472, by the Draconis Combine.
- Addendum II was passed unilaterally by the Star League in 2579 and rescinded the Ares Conventions.
Although noble in intention, the spirit of the Conventions was violated within a decade.
When the Capellan/Taurian border conflict escalated into the Rim War under Aleisha's successor Arden Baxter, the Confederation liberally used nuclear and chemical weapons against Concordat worlds, as Baxter considered the Conventions non-binding when applied to a non-signatory nation. Similarly, civil wars were not covered which caused conflicts like the Davion Civil War to be much more damaging than wars between states. 
Nevertheless, the Conventions were by and large upheld, with no little success: less people died in all conflicts between parties who adhered to the Conventions than during the Fourth Succession War. However, proving that the fears of the United Hindu Collective were well-founded, the Conventions had now effectively legitimized warfare. Soon, petty grievances were fought out on battlefields and wars that had begun dying down escalated again. The now-constantly shifting political borders necessitated additional legislation building on the spirit of the Conventions. Soon, laws were drafted which made the population of newly conquered worlds automatically eligible for citizenship in the state of their conquerors. Among the first those laws was the Liberation Act of 2448, enacted by the Free Worlds League.
Acting in accordance to the Conventions, the Terran Hegemony was regularly asked to participate as a neutral party in observatory boards and inquiry commissions, a position which would lay the groundwork for Ian Cameron to begin work on the Star League Accords. On the flipside of the coin, once the Star League had been established, disarmament of militaries which had been on a war footing for more than a century left millions of soldiers disenfranchised and restless. Staving off the economic problems of sudden peacetime economies and preventing armed conflicts between the former soldiers and the SLDF were one of the reasons for the Reunification War.
In 2579, as part of Ian Cameron's declaration of war, the Star League unilaterally passed Addendum II, rescinding the Conventions. Despite protests, and the strict adherence of the Free Worlds League to the Conventions during the war against the Magistracy of Canopus, they were never reinstated. The many conflicts the SLDF participated in - such as the Hidden Wars or the Amaris Civil War - were fought as total wars.
In the centuries after the Fall of the Star League, the articles of the Ares Conventions were used as shorthand for "civilized warfare", while two different codes of conduct replaced them.
Sheer necessity caused the development of an informal code called "Honors of War" during the Third Succession War. This code, based on the Ares Conventions, was created in an age of ever-decreasing technological knowledge and manufacturing capability, necessitating a code of conduct which would ensure the survival of both knowledge and the increasingly irreplaceable 'Mechs and JumpShips. While created with different aims and lacking the means for oversight and less violent conflict resolution, both were successful in preventing further massive civilian death tolls. 
The Clans, meanwhile, developed Zellbrigen, an honor system which is similar to the Conventions in many ways, especially by emphasizing ritualized, sanitized warfare up to the point of conflict resolution by personal combat. However, Zellbrigen differs from the convention in the casual disregard of infrastructure and non-military personnel: they are seen as resources not to be wasted, as opposed to civilians who need to be protected. 
- While some earlier sourcebooks listed the Magistracy of Canopus and the Outworlds Alliance as being among the ten states invited to the Ares Conference, this was clearly an error, as neither state existed at the time. Era Digest: Age of War noted this mistake and corrected it by naming the Principality of Rasalhague and the United Hindu Collective as the final two invitees.
- Era Digest: Age of War, pp. 13-15 (pp. 14-16 PDF)
- Handbook: House Marik, pp. 30-31
- Era Digest: Age of War, p. 12 (p. 13 PDF)
- Handbook: House Marik, p. 18
- Handbook: House Liao, pp. 18-19 (pp. 20-21 PDF)
- Mercenary's Handbook: 3055, p. 7
- The Periphery (1988), p. 20 (p. 19 PDF)
- Era Digest: Age of War, p. 5 (p. 7 PDF)
- Handbook: House Marik, p. 19
- The Star League, pp. 42-43 (pp. 44-45 PDF)
- Historical: Reunification War,p. 21 (p. 23 PDF)
- Mercenary's Handbook: 3055, pp. 8-9