Abduction of Lyanna Stark

"My sister loved him. She bore his children. Swaddled them, rocked them, fed them at her own breast. Elia wouldn't let the wet nurse touch them. And beautiful, noble Rhaegar Targaryen... left her for another woman."

- Oberyn Martell

The Abduction of Lyanna Stark was one of the events that led to the outbreak of Robert's Rebellion.

Abduction
For unknown reasons, the heir to the Iron Throne, Prince Rhaegar Targaryen, abducted Lyanna Stark, the daughter of Lord Rickard Stark, Lord of Winterfell and Warden of the North and hid her away in Dorne. This event was extremely scandalous as Lyanna was the daughter of the Warden of the North and betrothed to Lord Robert Baratheon, the Lord Paramount of the Stormlands, while Rhaegar was married to Princess Elia Martell, with whom he had two children.

Aftermath
After Lyanna's abduction, her older brother Brandon, who had been on his way to Riverrun to wed Catelyn Tully, rode instead to King's Landing demanding that King Aerys II Targaryen have Lyanna returned and Rhaegar punished. He was taken prisoner and charged with treason, at which point his father was summoned to ransom him. Aerys' councilors urged him to pardon Brandon Stark, as Rhaegar had stolen his sister and was the heir of the Warden of the North. The Mad King then had both of them put to death.

In the books
In the A Song of Ice and Fire novels, Lyanna and Rhaegar first encountered each other at a tourney at Harrenhal in the Year of the False Spring. The details of this are revealed in the story of the Knight of the Laughing Tree, which Meera Reed tells Bran Stark as they make their journey to the Wall.

Lyanna attended the tourney with her brothers. They met Howland Reed for the first time when Lyanna rescued him from a group of bullying squires. The tourney was attended by several important figures such as King Aerys II Targaryen, Prince Rhaegar and his wife Elia, Lyanna's betrothed Lord Robert Baratheon, Lord Mace Tyrell, Prince Oberyn Martell and Lord Jon Connington. During the feast Rhaegar, a noted musician, performed a sad and beautiful song that made Lyanna weep; when her younger brother Benjen teased her for crying, she poured wine over his head.

Rhaegar went on to win the tourney, unseating Lyanna's older brother Brandon, Lord Yohn Royce, Ser Arthur Dayne and Ser Barristan Selmy. Then the victorious Rhaegar urged his horse past his Elia, laying the blue winter rose laurel in Lyanna's lap and declaring her the Queen of Love and Beauty, to the shock of everyone present.

A year after the tournament, Lyanna was kidnapped by Rhaegar with the help of Arthur Dayne and Oswell Whent. Rhaegar carried her off and raped her. Barristan Selmy is convinced that "Prince Rhaegar loved his Lady Lyanna," though that love sent thousands to their respective deaths. According to Viserys Targaryen, Rhaegar was simply not happy with his marriage. If Rhaegar was happy with Elia, he would not have needed Lyanna. He once blamed their sister Daenerys Targaryen for being born too late to marry Rhaegar. A happily married Rhaegar would need neither Elia nor Lyanna. Daenerys countered that, by the same logic, it was the fault of Viserys that he was not a girl, because then he would be the sister destined for Rhaegar. Viserys beat her cruelly for her insolence. Cersei Lannister, who was infatuated with Rhaegar, believes that the madness led King Aerys to deny Lord Tywin's proposal of marriage between her and the crown prince; Cersei thinks that if she herself had only married Rhaegar as the gods had intended, he would have been satisfied in his queen and never have looked twice at the "wolf girl".