In 2375 Worf killed Chancellor Gowron in combat after Gowron risked the war for his own glory, and Martok became leader of the Klingon Empire.
In 2375 Gowron grew jealous of General Martok's success and took personal command of Klingon forces, ordering attacks that wasted ships and soldiers. Ezri Dax pointed out to Worf that the Klingon Empire was dying through corruption and that Worf could change it.
Worf challenged Gowron and killed him in a duel. As the winner he could have become chancellor, but he gave the cloak of office to Martok, a respected soldier from a common background.