The 2344 battle in which the Enterprise-C was destroyed defending a Klingon outpost from Romulans, a sacrifice that helped bring the Klingons and Federation together.
In 2344 the Klingon outpost on Narendra III came under attack by Romulan warbirds. The Federation and the Klingons were not yet allies, but the USS Enterprise-C under Captain Rachel Garrett answered the outpost's distress call and went to its defence, though it was heavily outnumbered. The Enterprise-C was destroyed in the battle.
The Klingons were deeply impressed that a Federation crew had died to protect Klingons, and the sacrifice helped lead to the close alliance of the two powers by the 2360s. The story is told in the Next Generation episode Yesterday's Enterprise.
In that episode, the Enterprise-C is pulled through a temporal rift into 2366 just before its destruction, creating a history in which the Federation is losing a war with the Klingons. The crew chooses to return to the battle to restore history. Tasha Yar goes with them and is captured by the Romulans, and her daughter Sela later becomes a Romulan commander.