In mid-2371, General Martok was replaced by a Changeling while hunting saber 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. The Martok Changeling, even at this stage, was also hoping for a conflict between the Federation and the Klingon Empire; to this end, he had one of his commanders (Kaybok) executed for refusing to fire on the USS Defiant.
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 lasting from 2372 to 2373. (DS9: "The Way of the Warrior")
The real Martok was informed of the Martok Changeling's actions by Julian Bashir, Worf and Elim Garak in Internment Camp 371 later that year, remarking it was "a grave dishonor". (DS9: "In Purgatory's Shadow")