The Path of the Unwritten.
The world is a dangerous place, and moving between the relative safety of towns and cities is a hazardous occupation not undertaken lightly. Because of this, long distance trade is a difficult and expensive enterprise. Several generations ago, a group of merchants banded together and hired a mercenary group to protect their trade caravan as they made the long, dangerous journey between distant cities. The protection offered by the fighters allowed for the merchants to ensure less of their stock was lost and for them to travel to previously isolated townships, generating huge income and quickly raising them to a prominant status.
The Wagonmasters Guild was officially created a few years later with an official agreement between the leader of the traders (at the time a charismatic dwarf named Simein Fortright), and the head of the mercenary band (a battle scarred brute of a dragonborn named Garrus).
Since that agreement, the guild has gone from strength to strength and now weilds serious political and economic power in many of the cities and towns it operates in.
Tired of the politics (and some say dissapointed by what the guild had become), Guildmaster Fortright stepped down from the leadership and retired, along with several of his closest friends, also high-ranking members. Into the vacuum of power that was created by this desertion stepped Madame Florish, an already wealthy and very cunning Tiefling matron, who quickly and quietly filled the ranks with her extensive family. Since her rise to power, Madame Florish has expanded the military might of her organisation and solidified their political powers in the majority of the major cities.
The guild controls a large percentage of overland trade, operating several huge trade caravans under the protection of a defensive guard big enough to dwarf even some of the larger cities standing armies.