As a young elf, Sylvanas showed great promise in marksmanship and trained as a ranger alongside her sisters Veressa and Alleria. However unlike her sister she also had a strong talent for magic. She was one of the few Sorceror-Rangers in Quel'Thalas who studied not only under the finest rangers in the elven kingdoms, but also the most powerful mages in Dalaran. She was tutored by Khadgar as well as Medivh at a young age until she took up tutelage under the mages of Silvermoon. With the influence of Khadgar, she soon learned that power was the most important goal for anyone. With power she could mold Quel'Thalas into what she saw fit. During her years as a ranger, she lead many attacks against the trolls of Zul'Aman. Secrely however, she confiscated the Troll shamanistic and magical artifacts for her own studies. Also because the High Elves relied heavily on the Sunwell to quench their thirst for magic, Sylvanas sought more powerful from it on a daily basis.
Sylvanas had much competition for power. Her main competition came not only from the Royal Family, but also her own sisters. To gain full access to the Sunwell, she had to eliminate all those who were in her way. She was ambitious to say the least in skillfully maneuvering herself into a position of trust with the King. It was rumoured that she even had desires upon Prince Kael'thas so she could marry herself into the royal family.
With the Empire of Stormwind in the south growing more and more powerful and seeing the potential for an enemy that she could deal with, she initiated secret contact with the Emperor. Sylvanas told him, that she would give them the strategic locations of all the High Elven forces in Quel'Thalas, if she could become the ruler of the Elven lands, with submission to the Empire. Years of negotiations took place, with the Emperor ceding Quel'Thalas to her at least in name, while she worked to keep the Elves isolated in the north.
When the Empire's forces spread from the north to the south, Sylvanas, now a trusted advisor to the King, advised that her sisters take as many people away from the city as possible, to get to Theramore and the Dark Portal that Medivh had opened. Though Sylvanas was ruthless she didn't have an interest in seeing her people destroyed, only to consolidate power for herself. She secretly withheld the location of Theramore and the portal from the Emperor, effectively playing both sides. When her sisters left with their forces, Sylvanas ordered all of those who were not loyal to her, out of the city to defend the outer regions of Quel'Thalas knowing that the Empire's forces were ready to attack. Consolidating her own supporters inside Silvermoon, when the Empire's forces attacked the city from the air, she took her opportunity and assassinated the King. She placed the prince under house arrest, and did not inform him of his father's death. The last action that she took in Silvermoon, was secretly smuggle out 30 vials of water from the Sunwell, then ordered it destroyed, temporarily cutting off the High Elves from the powers of the well.
Secretly, she and her supporters, and the Prince escaped to Sunstrider Isle while the Empire's forces attacked the city. She later "negociated" a truce with the Empire's forces. Her sisters, unaware of her betrayal were entirely unaware of what happened to their homeland since communications were cut off.
The Emperor followed the agreement, and appointed Sylvanas Windrunner as Viceroy of Quel'Thalas. With a High Elf leading the High Elven lands, the Imperial Advisors believed that it was in their best interest to have Sylvanas in charge with that region. However they also appointed to oversee her, Marshal Heinrich Dimler was appointed with a contengient of Loyalty Ministers to act on behalf of the Empire within Quel'Thalas. Nontheless Sylvanas has maintained the power that she had held before, secretly creating another Sunwell on Sunstrider Isle and heavily forbidding any human to enter that area.