Turkana IV was the inhabited fourth planet of the Turkana system and the site of a failed Federation colony. This non-aligned planet was the site of an Earth colony. Turkana City was one of the settlements on this planet.
The planet's government began breaking down in the 2330s. Dozens of factions developed, and civil war broke out. The Turkana government gave emergency powers to the two largest factions, the Coalition and the Alliance, but it was quickly overthrown by those cadres, and the planet broke away from the Federation in the 2350s, the two factions declaring the planet's independence. Lawlessness became the norm, and rape gangs became a common threat. For some of the citizens, drugs became an escape from the poverty and violence that they had to face every day. Few citizens escaped the planet. One of those fortunate enough to do so was Tasha Yar, who left the planet at age fifteen, and later went on to become a Starfleet officer. (TNG: "The Naked Now", "Where No One Has Gone Before", "Symbiosis")
Six years later, the USS Enterprise-D visited Turkana IV, in response to a distress call from the freighter Arcos. The Arcos was lost, but survivors crash-landed on the planet in an escape pod. At that time, the surface structures had all been destroyed by years of civil war, and the planet's inhabitants lived in structures that extended up to three kilometers below the surface. Proximity detectors were used to separate the two factions, and both sides' bases were heavily defended in order to prevent all-out conflict. (TNG: "Legacy")