Robert’s Rebellion was a 2-year war in revolt of the Mad King Aerys II, led by Robert Baratheon, Ned Stark and Jon Arryn. In the GoT series, the rebellion has already taken place before the show starts, so we do not get to see it, though it is constantly referenced. It was extremely significant as it resulted in the murder of the Mad King and the end of the 300-year Targaryen dynasty, with Robert taking the Iron Throne as the first ever non-Targaryen king of Westeros.
After many years of cruel and oppressive rule from the Mad King, most notably burning people alive, many houses began to speak out against him. The catalyst for the Rebellion was when Rhaegar Targayen, the Mad King’s son and Daenerys’ older brother, kidnapped Lyanna Stark, Ned’s sister and Robert’s bride-to-be. Brandon Stark, Ned’s older brother and heir to Winterfell, rode to King’s Landing with a small company and called for the death of Rhaegar. Upon arrival, the Mad King imprisoned Brandon and the 4 others, one of whom was heir to the Eyrie. The Mad King demanded their fathers ride to King’s Landing to answer for their sons. Lord of Winterfell, Rickard Stark, arrived at King’s Landing to the aid of his son Brandon, only for the Mad King to call for his and the others’ execution. When Rickard demanded for trial by combat, the Mad King burned him alive while his son Brandon was forced to watch, until he himself was strangled to death.
The Mad King demanded the heads of Ned Stark and Robert Baratheon from Jon Arryn, a father-like guardian to Ned and Robert. Instead, Jon Arryn, Lord of the Eyrie, raised his banners in revolt against the Mad King. Ned rode North to Winterfell to raise the banners of Winterfell and other Northern houses, while Robert rode to Storm’s End to raise his own. Following the marriage of Catelyn Tully to Ned and Lysa Tully to Jon Arryn, House Tully joined the forces of House Arryn, Stark and Baratheon. Once united, Robert Baratheon was the decided leader, as House Baratheon descended from House Targaryen, giving him the most legitimate claim to the throne.
The Mad King rose his banners, which included House Tyrell, but not House Lannister, of which Lord Tywin had recently resigned as Hand to the King. Robert and Ned won several battles on their march towards King’s Landing. The final and most decisive battle before the sack of King’s Landing was the Battle of the Trident. Prince Rhaegar and his Targaryen forces met the rebels on the Trident, where Robert slew Rhaegar with his warhammer, sending the rest of the Targaryen forces fleeing.
The Rebellion ended with the sack of King’s Landing, led by Lord Tywin Lannister. Knowing that the defeat of the Mad King was imminent after the Battle of the Trident, Lord Tywin, who had remained neutral up until this point, rode to King’s Landing in defense of the Mad King. But, when King Aerys II opened the gates of the city to Tywin, the Lannister army sacked the city and slew the Targaryens in the name of Robert. When the Mad King ordered Jaimie to kill his own father, Tywin, Jaimie instead put his sword through the back of the Mad King and was henceforth known as the Kingslayer. Lord Tywin sent one of his knights, Gregor Clegane, also known as The Mountain, to execute the rest of the Targaryen family. The Mountain raped and murdered Rhaegar’s wife, Princess Elia Martell of Dorne, and then killed their two babies, Aegon and Rhaenys.
When Ned arrived to King’s Landing, an argument ensued between he and Robert over the barbaric acts that were carried out in Robert’s name. This led to Ned fleeing King’s Landing. He rode south towards Storm’s End to finish the war on his own. At Storm’s End, the forces of Mace Tyrell, who had been fighting on the side of the Mad King, had Stannis Baratheon and his men surrounded in their castle, preventing the influx of food and causing those in Storm’s End to starve near death. Davos Seaworth saved Stannis and the others in Storm’s End from starvation by smuggling a load of onions into the castle, forever earning himself the nickname, The Onion Knight.
During the sack of King’s Landing, Queen Rhaella, sister-wife of the Mad King, was able to flee to Dragonstone with her youngest son, Prince Viserys. That night, a great storm swept across the land and Queen Rhaella died giving birth to Daenaryes Stormborn. Before any harm was brought to them, Daenarys and Viserys are sailed across the sea to Braavos, where they will wander in exile for several years to come. Many years later, Prince Viserys, will marry his sister to Khal Drogo in the hopes of raising a Dothraki army to help him reclaim the throne. He would eventually be killed by Drogo and Khaleesi will go on to raise an army of her own in her conquest of the Seven Kingdoms.