USS Enterprise
NCC-1701-C
This was the fourth starship Enterprise. It was Ambassador class, and 526 metres long. It was presumed destroyed when it was slaughtered in a vain attempt to save a Klingon Outpost from a Romulan attack. However, the ship was not destroyed. It traveled through a time rift just before its imminent destruction, and met the Enterprise-D.
In an extremely brave act, the surviving crew of the Enterprise-C and her captain, Rachel Garrett, travel back in time to the battle and are destroyed. This act of loyalty to the Klingons was what ended the years of hostility between the Federation and the Klingons. The Enterprise-C was the reason that things are the way they are today in the 24th century.