Robert's Rebellion

Robert's Rebellion was a civil war amongst the Great Houses of Westeros. It displaced the royal house Targaryen and saw most of its members wiped out, with only two survivors who fled into exile across the sea. House Baratheon became the new royal house after the war's conclusion.

In the books
In the Song of Ice and Fire novels, this civil war took place between sixteen and fourteen years before the beginning of A Game of Thrones. It lasted almost two years and began when Prince Rhaegar Targaryen kidnapped Lyanna Stark. Lyanna's betrothed, Robert Baratheon, and her brother Eddard Stark raised their banners in rebellion against this unjust act. House Tully and House Arryn joined them, whilst House Tyrell and House Martell (reluctantly, in the latter case) supported the Targaryens. House Greyjoy and House Lannister sat out most of the war.

The rebellion climaxed with the Battle of the Trident, when the rebel armies destroyed the royalist host and killed Prince Rhaegar. Lannister forces, apparently having decided to support the king, arrived at King's Landing to help defend the capital, but then turned on the Targaryens, brutally sacking the city and killing King Aerys II and Rhaegar's children. Aerys' other heirs, Viserys and Daenerys Targaryen, fled into exile across the sea. In the aftermath of the war it was discovered that Lyanna Stark had been killed, so Robert married Cersei Lannister in thanks for her father delivering him victory.

In the books the war is also called the War of the Usurper by Targaryen loyalists.