Warning: The following contains spoilers for Game of Thrones season 7
Robert’s Rebellion was based on a lie. As the seventh season of Game of Thrones has made clear, the alleged abduction of Lyanna Stark was actually an elopement and lawful marriage. Like with Helen of Troy, however, that elopement launched a thousand battles and forever altered the landscape of the Seven Kingdoms.
Though King Aerys deserved his title as the Mad King, it seems that his son, Rhaegar Targaryen, should bear a portion of the responsibility for driving Westeros into civil war. The call of his heart drove him to pursue an extramarital relationship that would bore a hole in the peace of Westeros. Had he stayed loyal to his first wife, Elia Martell, and their children, it's unlikely any conflict would have ever materialized.
As for Rhaegar's father, Aerys wasn’t always a bloodlust-filled lunatic. He was once an ambitious, sane, and competent ruler. With the help of Tywin Lannister as his Hand of the King, he did a remarkable job of keeping the Seven Kingdoms safe and prosperous. Though he grew peeved when Tywin seemed to receive all the credit for his success, Aerys allowed the Lannister leader to continue his work uninterrupted (though on occasion he would drunkenly insult Tywin’s wife, Joanna).
The Mad King’s descent began after he was kidnapped by Lord Denys Darklyn of Duskendale. Though he was held captive for just six months before Ser Barristan Selmy saved him, the seeds of paranoia were firmly planted in Aerys’ mind as he wondered how his own men could let him rot in another man's prison for half a year. The Mad King slowly lost trust in everyone and became a hermit, sealing himself in the Red Keep for months on end.
Only for the famed Tourney at Harrenhal did Aerys finally venture out. Like the Olympics of the Seven Kingdoms, this ballyhooed jousting event attracted all the big names in Westeros. Though it should have been a show of sportsmanship and honor, the Tourney set in motion the events that would lead to Robert’s Rebellion.
With his proud father looking on, Rhaegar Targaryen was pronounced the champion jouster at Harrenhal. As the tradition allowed, the victor would then declare his “Queen of Love and Beauty” before the crowd. Though the winner would usually nominate his spouse, Rhaegar Targaryen ignored his wife, Elia Martell, and instead gave the beautiful Lyanna Stark “a crown of winter roses, blue as frost." As Ned Stark remembers in A Song of Ice and Fire, this was “the moment when all the smiles died" amid the concerned crowd.
It was an unexpected move from Prince Rhaegar, whose silvery hair and good looks made him a heartthrob to every woman in his wake. Though they cheered him on in his victory, they quickly grew silent at his show of affection for the woman who wasn’t his wife. None of this was lost on Robert Baratheon, who was set to make Lyanna his bride.
Naming Lyanna Stark the Queen of Love and Beauty would spell death for Rhaegar, and war for the Seven Kingdoms. Not long after the tourney, word spread that Rhaegar had kidnapped Lyanna and disappeared with her. This enraged the leaders at Winterfell and spurred Lyanna’s brother, Brandon Stark, to ride to King’s Landing and demand Rhaegar be held responsible for his wanton actions.