(covers information from several alternate timelines)
The Battle of Narendra III was an attack on a Klingon outpost on Narendra III by four Romulan warbirds and subsequent defense of the outpost by the Federation Ambassador-class starship USS Enterprise-C in 2344. The conflict had a major impact on interstellar politics in the Alpha and Beta Quadrants.
By the mid-24th century, relations between the Romulan Star Empire and the Klingon Empire had deteriorated significantly. The Romulans began to view the Klingons as barbaric savages, while their former allies saw them as cowardly and without honor. Treaties were abandoned, and the two powers grew to despise one another.
This hostility reached a boiling point in 2344, when four Romulan warbirds decloaked and began an orbital bombardment of the Klingon settlement on Narendra III. The poorly defended colony soon fell prey to the Romulans' surprise attack, and issued a distress call. (TNG: "Yesterday's Enterprise")
Despite tensions between the Klingons and the Federation, the Enterprise intercepted the distress signal and proceeded to the Narendra system at maximum warp. By the time the vessel arrived at Narendra III, the Klingon outpost had been almost completely destroyed, and although the Enterprise was equipped with state-of-the-art weaponry and defenses, it was simply no match for the combined destructive power of four Romulan warbirds. Realizing that defeat was inevitable, the crew nevertheless made a valiant effort to protect the Klingons, engaging the warbirds in a fierce battle. After several long minutes of intense fighting, the Enterprise eventually succumbed to its wounds and was destroyed. (TNG: "Yesterday's Enterprise")
During the battle, the detonation of a photon torpedo created a temporal rift, which threw the Enterprise twenty-two years into a future in which the Federation and the Klingon Empire were at war. The displacement of the Enterprise had created an alternate timeline, in which the Klingons interpreted the starship's disappearance as a cowardly withdrawal and prompted a rapprochement with the Romulans, with Federation-Klingon relations rapidly breaking down. Upon realizing that their defense of Narendra III – though doomed – would convince the Klingons that the Federation was "honorable," the USS Enterprise-C returned to 2344 through the rift, shepherded by the USS Enterprise-D, and the proper timeline was consequently restored. (TNG: "Yesterday's Enterprise")
Not all members of the Enterprise-C's crew were killed in battle. There were rumors of survivors taken prisoner by the Romulans, although there was no evidence of this until 2368. In that year, Romulan Commander Sela confirmed that she was the daughter of Tasha Yar, who had accompanied the Enterprise back to Narendra, had been captured by the Romulans, and had become the consort of a Romulan general. (TNG: "Yesterday's Enterprise", "Redemption II")
The battle had significant political repercussions. With the Klingons disgusted by the Romulans' "cowardly tactics," the conflict caused their relations to deteriorate even further. It also drove the Klingons and the Federation much closer together, as the Klingons were impressed with the "honorable sacrifice" of the Enterprise and its crew. (TNG: "Yesterday's Enterprise")