Jaehaerys Targaryen's uprising

In the late reign of King Maegor Targaryen, his nephew Prince Jaehaerys was declared the rightful king by House Baratheon. More than half of Westeros participated in this rebellion.

Prelude
After the death of King Aenys Targaryen, Prince Maegor returned from exile and demanded the Iron Throne. Some time later during the Faith Militant uprising, his nephew Aegon, Prince of Dragonstone, raised an army of westermen and marched on King's Landing. Before Prince Aegon could get there in time, Maegor ordered his bannermen to attack the rebel prince's host from all sides while he mounted Balerion the Black Dread. Maegor's army met Aegon's beneath the Gods Eye, easily defeating the significantly larger host by killing Aegon and his dragon Quicksilver, crippling the army's morale. Thus ended the uprising for the rightful heir. His siblings, however, Jaehaerys included, still remained.

Uprising
In the final days of Maegor's reign, Lord Baratheon declared Prince Jaehaerys Targaryen the rightful king, and Houses Lannister, Tyrell, and Arryn soon followed. The lord admiral, Daemon Velaryon, gave the Royal Fleet to Prince Jaehaerys, Queen Rhaena took off on Dreamfyre, and Grand Maester Benifer fled the capital. The rebellion was short-lived, as Maegor was found dead on the Iron Throne with his wrists cut and one of the swords pointing through his neck.

Aftermath
After the uprising, Prince Jaehaerys was crowned King Jaehaerys I. He made several reconciliations with enemies made by Maegor and Aenys, quickly earning the name "Jaehaerys the Conciliator".

In the books
In Fire & Blood, this uprising is the same.