The fall of the Federation colony on Turkana IV into civil war and rule by rival armed factions, the lawless setting of Tasha Yar's childhood, which ended with the colony cutting its ties to the Federation.
Turkana IV's government began to fall apart in the 2330s. The colony split into armed cadres, and the authorities gave them police powers in the hope of keeping order. The factions instead destroyed the government, and two of them, the Coalition and the Alliance, were left fighting over underground tunnels while the surface cities lay in ruins.
Tasha Yar was orphaned there at five and spent her youth caring for her sister and evading rape gangs, as she recalls in The Naked Now and Where No One Has Gone Before. She escaped at fifteen and joined Starfleet. The colony later declared its independence and threatened to kill any visitors.
The Enterprise-D goes there in the episode Legacy in 2367 to rescue a freighter crew and meets Tasha's sister Ishara, who uses the crew's trust for her faction's ends.