The Martok Changeling in 2372.
In mid-2371, General Martok was replaced by a Changeling while hunting sabre bear along Kang's Summit on Qo'noS. Following his capture, the "real" Martok was sent to Internment Camp 371. (DS9: "In Purgatory's Shadow")
In early 2372, the Changeling Martok tried to weaken the Alpha Quadrant by pushing Chancellor Gowron into believing that the coup on Cardassia Prime was influenced by the Dominion. After leading a fleet to Deep Space 9, the Klingons invaded Cardassian space.
Martok and Gowron later led the Klingons into battle against Deep Space 9, that eventually led to a year-and-a-half conflict between the Klingon Empire and the Federation in a conflict known as the second Federation-Klingon War. (DS9: "The Way of the Warrior")
In early 2373, Martok was discovered to be a Founder, and was killed at Ty'Gokor when discovered. ("Apocalypse Rising") Unfortunetly, this did not end hostilities between the two governments, as the Federtation would still not allow the Klingons to annex the Archanis sector. They would not be reunited until the Cardassians joined the Dominion later that year. (DS9: "By Inferno's Light")