In 2257 the Federation-Klingon War ended at Qo'noS when Michael Burnham gave L'Rell the means to unite the Klingon houses under threat of destroying their homeworld.
By early 2257 the Klingons had overrun much of the Federation and were threatening Earth itself. Starfleet, desperate, accepted a plan from Philippa Georgiou, the former Terran Emperor, and sent Discovery to Qo'noS. Officially the mission was to map the planet's defenses, but Georgiou secretly intended to destroy the Klingon homeworld with a hydro bomb.
Michael Burnham discovered the plan and refused to let Starfleet commit genocide. She persuaded Admiral Cornwell to change course and offered a different solution: the bomb's detonator would go to L'Rell, who could use it to compel the feuding Klingon houses to follow her.
L'Rell took the detonator, confronted the great houses and declared herself chancellor, ending the war. Georgiou was released and left, Ash Tyler went with L'Rell to Qo'noS, and Burnham was reinstated in Starfleet and honored for upholding Federation principles.