Mr. Laser Beam wrote:
^ Incorrect. Tasha and the Ent-C travelled into the past of the *same* timeline. But since the Ent-C's sacrifice was already a part of history, they didn't create an alternate timeline; merely fulfilled the existing one. (Nero and Spock Prime, on the other hand, created a branching timeline, since their actions were not a part of history.)
No. When the Enterprise-C fell trough that time-rift a new timeline/universe was created. One in which Armus never killed Tasha (and Data's head never ends up in a San Francisco cave).
We get a short glimpse into that universe, since we basically follow the course of the Enterprise-C from the view-point of the Enterprise-D.
When the E-C returns to her own timeline/universe through that rift this war-with-the-Klingons-tl/u doesn't get wiped out. It continues on.
But we follow the E-C back to her own tl/u to the time we left it at the beginning of the episode. There all this has indeed already happened.
This means that when we first see Tasha in Encounter at Farpoint
her alternate-timeline-ego has already died on Romulus.