In an alternate timeline, the Second Federation-Klingon War was a major conflict between the Klingon Empire and the United Federation of Planets that took place in the mid-24th century following the deterioration of relations between the two powers. The timeline had been created when the USS Enterprise-C disappeared into a temporal rift in 2344 while defending a Klingon outpost on Narendra III from a Romulan attack. (See: Battle of Narendra III)
By 2366, the war was going badly for the Federation. Forty billion lives had already been lost and Starfleet Command believed that surrender was inevitable within six months. Although the Federation had emerged victorious in several battles, including one at Archer IV, Starfleet had lost over half the fleet, and all vessels were being pressed into service, regardless of age.
Two Enterprises meet
In that year, the USS Enterprise-D, a Galaxy-class warship, encountered a radiation anomaly near Starbase 105. The Enterprise-C emerged from this anomaly, which was later discovered to be a temporal rift. Captain Jean-Luc Picard, along with his personal confidante, the El-Aurian Guinan, who had somehow been able to "sense" the change in timeline, determined that the Enterprise-C's defense of the outpost was the pivotal point that most-certainly would have prevented the war. He convinced Captain Rachel Garrett to take the Ambassador-class vessel back through the rift in order to restore the intended timeline in spite of the near-certain prospect of the ship's immediate destruction.
The timeline (along with the war) ceased to exist following the return of the Enterprise-C to its own time. No one on the ship remembered the alternate timeline; however, one remnant of the timeline survived: the alternate Tasha Yar, who served as tactical officer of the alternate Enterprise-D and returned with the Enterprise-C into the real timeline. (TNG: "Yesterday's Enterprise", "Redemption II")
- It is not elaborated how a Romulan attack on a Klingon outpost led to war with the Federation, though it would be plausible that the Enterprise-C's disappearance during its defense of the outpost and the likely unfavorable Klingon reaction to such an occurence may have been a factor.
Another possibility is that war would have happened anyway, and the Enterprise-C's sacrifice impressed the Klingons into maintaining the peace.