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. Compliance was almost universal and reduced both the human and economic costs of war, unfortunately also enshrining it as a means of solving even the slightest disputes.
The Conventions were upheld until rescission during the military buildup leading to the Reunification War and formally renouncement at the beginning of the First Succession War. Despite no longer part of a binding treaty, the Ares Convention 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 had resorted to hit-and-run tactics and hiding in cities. After the conflicts had already badly damaged the planetary infrastructure and tens of thousands of civilians were causalities of the urban combat, both commanders resorted to using strategical ABC weapons. In the end, both Captain-General and Capellan Chancellor Aleisha Liao ordered their forces to withdraw, the planet was subsequently abandoned.
The conflict 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, with 320 pages in total. The Ares Conventions were signed by eight of the ten great nations (the Terran Hegemony, Capellan Confederation, Free Worlds League, Rim Worlds Republic, Federated Suns, Principality of Rasalhague, Lyran Commonwealth, and Draconis Combine) on the 13th June of 2412. 
While even some signatories were critical towards the Conventions, the Taurian Concordat and the United Hindu Collective, outright refused to sign them. The Concordat was distrustful of the Capellans in general and considered the Conventions hypocritical, having been in a border conflict with the Confederation for several years, with Liao forces conducting massacres similar in scale to Tintavel before, during, and after the Conference. The Collective meanwhile feared 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 bombardement except against vital military targets which were not anywhere near populated areas.
- Article III established the white flag with adorned red S as 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 explicitely disavowed combat in cities - unless a military target was within the city - and against civilian targets.
- Article VI forbade research, developement, 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 ridigy, these appendices created the modern definition of a pirate merely by omission, as a pirate was someone neither a 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 the fears of the Hindu Collective right, the Conventions had now legitimized warfare. Soon, petty grievances were fought out on battlefields and wars that had begun dying down lit up 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 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 not been off 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 conflict, 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, as well as the increasingly irreplaceable 'Mechs and Jumpships. While created in different aims and lacking in 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. 
- 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