The Guild of Mercantilers arose during the Black Powder wars in the 1800s. It was a conglomeration of nobles, merchants, and mages who wished to see an end to the fighting. The quickest way to end the conflict was to gain control of Soulstones, and they put all their energy to that task.
With control of the world's Soulstones largely in their grasp, they were able to force the world into a cease fire. The Breach reopening years later led to a growth in their power but also a chafing of the world's nations at the Guild's somewhat tyrannical oversight.
After the Battle of London, the Guild pulled back its forces around the world and is choosing its battles carefully, ensuring that it has influence with whatever nations and empires survive the bloody war.