The capture of Starbase 1 by the Klingon House D'Ghor in 2257, with the loss of about eighty thousand lives, discovered by USS Discovery on its return from the mirror universe.
Discovery returned from the mirror universe nine months after it had left and found that the Federation-Klingon War had gone badly. The Klingon houses were competing with one another in the scale of their attacks, and about a fifth of Federation space was lost.
In the episode The War Without, The War Within, Admiral Cornwell orders the ship to Starbase 1, about a hundred astronomical units from Earth, which housed tens of thousands of people. Discovery arrives to find the station marked with the emblem of House D'Ghor and no Federation life signs.
The loss of a base so near to Earth pushed Starfleet Command to approve the desperate plan against Qo'noS that the mirror Georgiou proposed.