Changes

Closed timelines occur when an alternate timeline is created, then eliminated through the actions of "external" agents. Typically, the participants within these timelines are unaware of any change, either upon the creation of the alternate timeline, or once the timeline reverts to its "true" form, unless shielded from those changes by temporally-sensitive materials (for example, [[chroniton]]s and [[temporal shield]]s). Since this is the case, most incidents of closed timelines go unreported, except by the [[Department of Temporal Investigations]]. Such timelines usually involve a cyclic aspect (action in the original timeline altering the past/future, creating an alternate timeline, an object/person unaffected by the temporal change traveling to/already present in the past/future to ensure the original timeline is re-established). Often in the case of alternate timelines, they are erased by actions by the people within them, such as when [[USS Voyager]] destroyed the [[Krenim weapon ship]] and erased it from history, or when the crew of [[Enterprise (NX-01)|''Enterprise'']] sacrificed themselves and the ship in an alternate future in order to destroy [[Interspatial parasite|parasites]] in the present Captain [[Jonathan https://memory-alpha.fandom.com/wiki/Jonathan_Archer Archer|Archer]]'s brain in the hopes that he'd be able to save the future, erasing that timeline.

*In [[2344]], the {{USS|Enterprise|NCC-1701-C|-C}} responded to a distress call from a [[Klingon]] [[outpost]] on [[Narendra III]], which was under attack by [[Romulan]] forces. In the "real" timeline the Enterprise saved the outpost and strengthened relations between the Federation and the Klingons. However, during the battle with the Romulans a [[Temporal rift]] opened and the ''Enterprise'', badly damaged and disoriented, traveled to [[2366]], in the presence of its successor, the {{USS|Enterprise|NCC-1701-D|-D}}. Because the ''Enterprise''-C was not there to save the outpost it was destroyed and the Klingons declared war on the Federation. For the next 22 years, the two powers engaged in a bitter war that cost 40 billion lives and left the Federation on the brink of defeat. In this timeline, [[Tasha Yar]] was still alive and serving as tactical officer aboard the ''Enterprise''-D in [[2366]].<br /><br />[[Guinan]] began to feel the change in time as "not right". She advised Captain Picard of the changes and eventually convinced him to send the ''Enterprise''-C back through the rift. Yar discovered through Guinan that she died in the other timeline and felt that she could help aboard the ''Enterprise''-C rather than where she felt she didn't belong. The Klingons attacked the two ships as the ''Enterprise''-C flew toward the rift, while the ''Enterprise''-D defended her. The ''Enterprise''-D was likely destroyed by the Klingons, but the ''Enterprise''-C made it through the rift and restored the timeline, erasing the war from history. <br /><br />However, during the battle with the Romulans, the ''Enterprise''-C was forced to surrender and prisoners, including the alternate Yar, were taken. Yar had a child with a Romulan and was later executed. The child, [[Sela]], became a [[commander]] in the Romulan military and encountered the ''Enterprise''-D several times.