Remember when the 24th century Federation was at war with the Klingons, and it was only a matter of months before defeat was inevitable? Remember when Tasha Yar wound up at the tactical station on the Enterprise C when it defended Narendra III? Remember when the Enterprise was ship of war, and not a ship of peace?
No one else on Star Trek: The Next Generation remembers any of that, because most of it was part of a horrifying timeline that was undone (and then re-done) by one crew doing the right thing at the right time. Guinan is the only person on the Enterprise D with any recollection of it — though the time-displaced Tasha Yar certainly had ramifications.
Confused? Naturally. It's all a part of the epic Season 3 episode of TNG, "Yesterday's Enterprise" where history speaks of one ship defending the helpless at a critical point. Years before Picard and company, the Enterprise C defended the Klingon outpost Narendra III from Romulan attack. The ship was destroyed, but this moment solidified peace between the Federation and the Klingons. Take that ship out of history by way of a convenient portal through time that whisks it away? You get a war, a horrible, horrible war... and Tasha Yar still alive.
Guinan feels that it's wrong, so she convinces Picard to send the Enterprise C back. They're going back to die, but they are also going to make sure that this war never happens. The one difference that all of this timey-wimey-shenaniganery will make is that this timeline's Tasha will be on the C, and her being there definitely had consequences.
All of this is explored in the new episode of SYFY WIRE's Warp Factor. Defending the helpless against insurmountable odds may lead to death, but it's what Starfleet does. Take a watch below.