The forced evacuation of more than fifteen thousand human colonists from Tau Cygni V, a world ceded to the Sheliak by treaty, which Data had to bring about alone.
In The Next Generation episode The Ensigns of Command, the Sheliak Corporate demands that humans be removed from Tau Cygni V within days, as the Treaty of Armens entitles it to do. The colony descends from a ship that went astray ninety years earlier, and hyperonic radiation rules out transporters, so only Data can go down by shuttle.
The colonists, led by Gosheven, are proud of having tamed a desert and will not leave. Data finds one ally, Ard'rian McKenzie, and fails with reasoning and persuasion. He finally adapts a phaser to work in the radiation and destroys the aqueduct, showing how helpless the settlers would be against Sheliak weapons.
In orbit, Picard buys time by searching the enormous treaty. He invokes its arbitration clause and names the Grizzellas, who are in hibernation for months, until the Sheliak grant three weeks.