Martok was a Klingon warrior who rose to the rank of general in the Klingon Defense Forces, and later became Chancellor of the Klingon High Council. His brilliant leadership during the Dominion War made him very popular both among other warriors and among the civilian population.

Fortune smiled on Martok. During a battle with the Romulans, Martok's ship was boarded. Martok acquitted himself so well in the fighting that he earned a battlefield commission.
(DS9: "Once More Unto the Breach")

From there, Martok worked his way up the ranks until he became a general. He played an important role in commanding the forces loyal to Gowron at the Battle of Mempa during the Klingon Civil War in 2368.
(DS9: "Once More Unto the Breach", "Apocalypse Rising")

By 2372, Martok had become an important military adviser to Gowron, and was instrumental in convincing Gowron to attack the Cardassian Union, starting the Klingon-Cardassian War.
(DS9: "The Way of the Warrior")

When imprisoned in Internment Camp 371 he and Worf forced to fight the Jem'Hadar in single hand-to-hand combat, as the Jem'Hadar were training for an invasion of the Alpha Quadrant and In mid-2373 he was joined by Worf and Garak.
(DS9: "Soldiers of the Empire")

Together, Martok's and Worf's actions in fighting the Jem'Hadar kept the Dominion soldiers occupied long enough for Garak to complete modifications to a secret transmitter that commanded a nearby runabout to beam the prisoners out of the camp.
(DS9: "In Purgatory's Shadow", "By Inferno's Light")

Following his escape from the prison camp they all returned to Deep Space 9.
(DS9: "By Inferno's Light")

Martok was initially too cautious in his command of the Klingon warship. This, combined with the crew's already-low morale , made for a dangerous situation aboard the Bird-of-Prey. Martok exacerbated the situation by deliberately avoiding opportunities to engage the Jem'Hadar in combat.

The situation came to a head when the crew nearly mutinied against Martok, but Worf changed the situation by challenging Martok himself.
(DS9: "Soldiers of the Empire")

After the Dominion War broke out, Martok, along with Worf, were instrumental in convincing Gowron to support Starfleet's offensive to recapture Deep Space 9 from the Dominion in Operation Return.
(DS9: "Favor the Bold", "Sacrifice of Angels")

Martok continued to serve as commander of the Rotarran. He was promoted to Supreme Commander of the combined Starfleet-Klingon Ninth Fleet in early 2374.
(DS9: "You Are Cordially Invited", "Once More Unto the Breach")

A few days after Operation Return, Martok sent several scout ships to the Cardassian border to check for signs of Dominion activity.
(DS9: "Resurrection")

Martok dispatched three squadrons of Klingon attack cruisers to the Dorala system's defense perimeter. Kira Nerys was confident that Martok would ensure the Klingons held the line until Starfleet reinforcements arrived. (DS9: "The Reckoning")

As commander of the Ninth Fleet, Martok led several of the most important engagements in the war, including the First Battle of Chin'toka, the Second Battle of Chin'toka, and the Battle of Cardassia.
(DS9: "Tears of the Prophets", "The Changing Face of Evil", "What You Leave Behind")

Martok gained immense popularity in the Empire for his near-heroic exploits against the Dominion. Gowron feared Martok's popularity.

In late 2375, Gowron presented Martok with the medal of the Order of Kahless, and immediately assumed personal command of Klingon forces on the front lines, relegating Martok to a secondary command role.
(DS9: "When It Rains...")

Gowron then began ordering Martok on near-suicidal missions against Dominion forces, hoping that a string of defeats would weaken Martok's popularity and discredit him as a military leader, or would die in battle.

However, Worf and Benjamin Sisko recognized that Gowron was jeopardizing the entire war effort to satisfy his political ego.

Martok refused to betray his loyalty to Gowron, despite the latter's treachery. Worf then challenged Gowron himself, accusing Gowron of being a coward for squandering the Empire's resources.

Worf killed Gowron in the ensuing combat, becoming the new chancellor. However, he immediately abdicated in favor of Martok. Martok tried to refuse, claiming he did not seek the leadership, but Worf responded by quoting Kahless: "Great men do not seek power, they have power thrust upon them."
(DS9: "Tacking Into the Wind")

Following the successful conclusion of the war, Martok returned to Qo'noS to assume the role of leading the Empire and finally purging it of the corruption that had so weakened it in the past.
(DS9: "What You Leave Behind")