It was an essentially unknown event, that is their vanishing from their "proper" place in history and winding up in the possible future that resulted from that - the Klingons and the Feds were at war instead of becoming true allies, the war had apparently been one of attrition since it lasted several decades, and the Feds were finally at the stage where they were getting whupped.
To use an example, it's like the original version of the Age of Apocalypse in X-Men, before it was retconned into being a parallel Earth and not a radically changed prime Earth. Xavier's son, Legion, decided that the greatest threat to his father's dream of peaceful relations between humans and mutants was Magneto, so he traveled into the past to kill him before he became a serious threat to Xavier's vision. He ended up in a time period before the two went their separate ways, with a group of X-Men traveling back to try and stop him.
Legion almost succeeded in his goal, but at the last minute the past version of Xavier leapt in front of Magneto and took the fatal blow, causing a massive change in how history was supposed to go. Legion no longer existed because Xavier died before his conception, and Xavier's premature death caused Apocalypse to began his activities earlier than in the original history. Magneto took up Xavier's dream and founded the X-Men in the new reality, but by the time of the new "modern" era, Apocalypse's forces controlled much of the world and many of the characters in the X-universe were in considerably different places in this timeline. Some heroic characters were villainous allies of Apocalypse, while some enemies had never gone bad or had a common cause with the X-Men. Bishop was the only person, being from an alternate future himself, who remembered what the proper timeline was supposed to be and he went back into the past to stop Legion's actions. That supposedly reset the timeline to what it should have been, and ensured the X-Men their correct future.
Then the writers decided they wanted to do more stories in the AoA setting, so it was made into a parallel timeline instead.