Silver Millennium
It is, perhaps, a little unclear as to where Sailor Saturn was during the time of the Silver Millennium - was she a Princess as we later found out that she was? Or was she simply brought about upon the demise of the Moon kingdom?
Either way, we do know her story as it pertains to the final moments of the Silver Millennium as told by the three Outer Senshi. After the attack of Queen Beryl and the following destruction of the Moon Kingdom (wow, a reference to a previous story arc!), Sailor Uranus, Sailor Neptune, and Sailor Pluto were watching from their planets, helpless to do anything.
It was then the talismans, the most powerful weapons of the outer planets, resonated like we had never seen before. We gathered together ... and our glowing Talismans summoned from the forbidden planet, the one who must never be awakened ... the last Sailor Scout. ~ Sailor Uranus, Neptune, and Pluto, Act 30
As soon as she arrived, the others knew that it was all over - Saturn's appearance means that the end of the world was near. Sailor Saturn wasted no time and dropped her Glaive, sending the Silver Millennium and (supposedly) the evil present there into oblivion. However, of course, her eradication of evil was not complete, and these incarnations of Chaos continued to live on to torment Earth in future years.
Later, in the Dream arc, when confronting Nehellenia, the issue of the Reborn Villains is addressed - firstly, they are all incarnations of Chaos, and secondly, Saturn realizes what happened though she had destroyed the world.
Sailor Neptune: Your [Nehellenia's] pathetic curse did not kill the kingdom nor the Princess! It was destiny! You only wish you had that power!
Sailor Saturn: That's right. Tragedies kept happening, one after another, dragging our kingdom down. I thought I had extinguished it. To find out I missed a spot! ~ Act 41
So, though the Silver Millennium was destroyed and everyone was reborn on Earth, they were not meant to live in peace for long, because, ultimately, Earth was a chosen place.