From Halopedia, the Halo wiki
Courier guilds are organized smuggler gangs based on the human Outer Colony of Aleria. In 2453, a drought devastated Aleria's economy and forced many Alerians to become smugglers in order to support themselves. Some courier guilds were opposed to the Mols'Desias mining union that united Alerian mining groups, as they believed the union took advantage of Aleria's failing economy by monopolizing the job market and forcing workers to work unreasonable hours.
After the Unified Earth Government abandoned Aleria shortly after the Human-Covenant War began due to financial concerns, Aleria became largely controlled by various courier guilds. The collapse of Mols'Desias in 2554 formed the foundation for more courier guilds. The courier guilds maintain a large fleet of slipspace-capable freighters, which are often used by smugglers on operations. Although the UNSC considers the Alerian courier guilds to be little more than mere smugglers and petty thieves, the guilds provide Alerians with a source of income and allow them to feed their families.
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