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 convincing Chancellor Gowron that a recent coup on Cardassia Prime was effected by the Dominion. After leading a fleet to Deep Space 9, the Klingons invaded Cardassian space. The Martok Changeling also attempted to incite conflict between the Federation and the Klingon Empire; to this end, he had Kaybok, one of his commanders, executed for refusing to fire on the USS Defiant.
Martok and Gowron later led the Klingons in an attack on Deep Space 9. He attempted to continue the battle even when the odds turned against the Klingons, but was overruled by Gowron. Nevertheless, the incident 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 at the internment camp later that year, remarking it was "a grave dishonor". (DS9: "In Purgatory's Shadow")