Its official name is A Compact Among the Civilized Nations, Concerning the Use of Magic in Battle, but sorcerers (and nearly everyone else) simply call it The Treaty. Signed in the ruins of Camac That Is, dated Year 3825 of the Pearl Throne (PT.3825, or SM.348, as years are reckoned in the Matriarchy), The Treaty forbids the employment of sorcerers in combat, both as sorcerers and as common soldiers.
The Treaty was first proposed by Ak’koyr in PT.3820, after a battle near the market town of Anlayt. The Northern Reach was threatening to overrun Anlayt, which would have left the road to Ak’koyr itself clear. Amon the Red, a sorcerer in Ak’koyr’s military, knew about the bones of a Firedrake nearby; in desperation, he awakened it and ordered it to destroy the Valiant Men of the North (the Reachers’ army). Not knowing the necessary binding spells, nor having pure motives, the dragon killed Amon and then wreaked havoc on both armies. With fighting forces depleted, the two countries called a truce and agreed to remove sorcerers from military service. (The cannon was a recent invention, which made sorcery in wartime less necessary anyway.)
Afterwards, both nations (especially the Northern Reach) championed the idea of a general worldwide ban on sorcery in battle. The Conclave of Sorcerers, whose numbers had begun to decline, embraced the proposal. Other nations were at least agreeable to the idea, and sent delegations to Camac to hammer out the details. The Conclave sent a delegation as well, and inserted a clause that allowed sorcerers to use magic to protect themselves or family members in any conflict. Another exception allows sorcerers to serve in non-combat roles; for example, calling the wind on a naval ship or aiding Healers. Still, the Conclave has since pursued a policy of putting the needs of all Termag above the needs of any nation or locale. Some folk consider the Conclave to be a de facto nation, whose population is scattered among other nations.
As combat magic was a large part of sorcery up to this time, The Treaty actually accelerated the decline of sorcery (rather than protecting the existing numbers, as the Conclave had hoped). Major combat spells were put aside entirely, while simpler spells were repurposed to peaceful use. In the modern age, new and old enemies are driving a renaissance in combat magic. An untrained boy, who awakened an ice dragon to defend his besieged town, triggered the renewed interest—but those stories are available on most eBook sites. :-)
Next: U is for: (The) Unfallen