A war between the Federation and the Klingons.
The Klingon War began after the clash at the Binary Stars and ran for about a year. T'Kuvma's death turned him into a martyr, and the Klingon houses, though divided, were united by hatred of the Federation. Starfleet in turn was convinced that the Klingons would not stop until the Federation was destroyed.
The war was fought on many fronts, with the Klingons using cloaking technology that gave them an advantage, and with Starfleet adapting by rebuilding its vessels. The USS Discovery, equipped with the experimental spore drive, was a major factor, and Michael Burnham, Captain Lorca and Lieutenant Stamets were among those in the thick of things. The mirror universe also came into play, complicating the strategy.
The conflict ended when Burnham halted a Starfleet plan to destroy the Klingon homeworld, and the Klingons agreed to a ceasefire, with L'Rell leading the effort to unify them. The war cost the Federation many lives and ships and made its leaders question their ideals. The changes in Starfleet and Federation attitudes in this period shaped the era that followed, including the missions of the Enterprise under Captain Pike.