Tau Cygna V is a planet ceded to the Sheliak by treaty, where a forgotten human colony had to be evacuated under threat of destruction.
Tau Cygna V belonged to the Sheliak under a treaty with the Federation, but a human colony ship had settled there anyway decades earlier. Radiation on the planet prevented transporters from working and made it seem uninhabitable, yet the colonists had adapted. In The Ensigns of Command the Sheliak demanded their removal.
Data was the only one able to work on the surface, and he set about persuading the stubborn colonists to leave. Picard bought time by invoking a clause in the treaty that the Sheliak had overlooked, and the colonists were evacuated before the Sheliak arrived.