From Halopedia, the Halo wiki
The city was established shortly after the Unified Earth Government colonized the planet, funded by biotech associations looking to exploit Andesia's rich ecosystem for life-extending pharmaceuticals and exotic foodstuffs. Following the Human-Covenant War, turf wars between Insurrectionist factions and their criminal allies, corrupt law enforcement personnel, and UEG peacekeepers ensured there would be no peace on the streets of Promesa.
As part of training for teams returning to abandoned Outer Colonies, War Games designers on the UNSC Infinity created two simulations of Promesa: one depicting it in a recent state and one in a theoretical state showing what could have happened to the city if the colony had succumbed to social collapse and famine during the Covenant War.
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