Why Robert's Rebellion was built on a lie: Bran's Game of Thrones season 7 episode 7 finale line explained

A crucial moment in Westerosi history

*This post contains spoilers for Game of Thrones season 7 episode 7 - 'The Dragon and the Wolf'*

Bran Stark and Sam Tarly pieced together the puzzle of Jon Snow's parentage, his legitimacy and his claim to the Iron Throne tonight, in a flashback that confirmed Jon is not a bastard and this his real name is Aegon Targaryen.

Sam (via Gilly in an earlier episode) was able to complete the picture for Bran, who was aware that Jon was the son of Rhaegar Targaryen and Lyanna Stark from his Tower of Joy flashback, but not that High Septon Maynard had annulled Rhaeger's marriage to Elia Martell before remarrying the prince to Lyanna.

Bran noted how this all meant that Robert's Rebellion, the civil war that set up the events of Game of Thrones, was based on a lie.

This is because - and you can be forgiven for not remembering as we never witnessed it on screen - a major catalyst in the war between House Baratheon (supported by houses Stark, Arryn, Tully, Lannister and Greyjoy) and House Targaryen (supported by houses Tyrell and Martell) was the supposed 'abduction of Lyanna Stark'.

It was widely believed that Prince Rhaegar, heir to the Iron Throne, kidnapped Lyanna, Ned's sister who was betrothed to Robert Baratheon.

The event was even more controversial as Rhaegar was thought to still be married to Elia Martell.

The Game of Thrones Wiki explains what went down after the "kidnapping":

'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, Rhaegar's father, have Lyanna returned and Rhaegar punished. He was taken prisoner and charged with treason, at which point his father, Rickard Stark, was summoned to ransom him. Aerys's councilors urged him to pardon Brandon Stark, as Rhaegar had stolen his sister and was the heir of the Warden of the North. However, the Mad King, in his delusion and insanity, had them both put to death.

'The Mad King then had a raven sent to Lord Jon Arryn, the Lord of the Eyrie, Lord Paramount of the Vale, and Warden of the East, demanding that he hand over Robert Baratheon and Eddard Stark, Rickard's second son and Brandon's younger brother, both of whom he had been fostering at the Eyrie. Instead, Jon Arryn rallied his bannermen and rose up in rebellion against the crown. He then sent Robert back to the Stormlands and Eddard back to the North to do the same. Robert's Rebellion had begun.'

The abduction kickstarted the war, then, though Lyanna in fact chose to run away with Rhaegar.

The episode also disclosed Jon Snow's real name - Aegon Targaryen. Aegon was also the name of Jon's older brother, who was brutally murdered by Ser Gregor 'The Mountain' Clegane during the Sack of King's Landing. If Jon sits on the Iron Throne before the show finishes, he will be King Aegon IV Targaryen.