The Setlik III massacre was a major event of the Federation-Cardassian War.
The Federation colony of Setlik III was believed by the Cardassian Union to be the staging ground for an assault on Cardassian space, and a strike force was dispatched to the colony to destroy it. However, no such military base existed, and the Cardassian troops attacked the civilian population, with over a hundred casualties. The Federation starship USS Rutledge, under the command of Captain Benjamin Maxwell, was the first vessel to respond to the colony's distress calls, though it was stationed some hours away. Upon arrival at Setlik III, the Rutledge deployed landing parties who were instrumental in saving the lives of the remaining colonists. (TNG: "The Wounded") Crewman Miles O'Brien became known as the "Hero of Setlik III" and was decorated after leading a group of two dozen men against a Cardassian regiment at the Barrica encampment. Among O'Brien's other achievements on Setlik III was saving the lives of thirteen men by using a field transporter for the first time. (DS9: "Paradise", "Empok Nor")
Among the casualties were Maxwell's family, as well as Rutledge crewmembers Will Kayden and Raymond Boone. Boone himself was killed by the Cardassians after being captured. He was replaced by a surgically-altered Cardassian operative. He was not revealed as an impostor until 2370. (DS9: "Tribunal")
In 2367, Glinn Daro acknowledged that the attack on Setlik III had been "a terrible mistake", as the Cardassians had erroneously believed it to have been the staging ground for a massive Federation attack. (TNG: "The Wounded")