A Klingon victory over the Romulans in 2271 in the region later known as the Briar Patch, won under the command of Kor and long celebrated in Klingon song.
Klach D'Kel Brakt is the Klingon name for a region of dense nebular gas. There, almost a century before the Dominion War, the Dahar Master Kor led a Klingon force to a famous triumph over the Romulans.
The battle is not shown. Kor reminisces about it in the Deep Space Nine episode Blood Oath, and again in Once More Unto the Breach, where the old warrior's mind wanders during a raid and he believes himself back in the fight, giving orders to long-dead comrades.
The Enterprise episode The Augments identifies Klach D'Kel Brakt as the Briar Patch, the same region in which the Ba'ku planet lies in Star Trek: Insurrection.