Assault on Dragonstone

"I was sent to the royal island stronghold at Dragonstone to deal with Viserys and Daenerys, the last surviving Targaryen children. Before I arrived, however, they escaped across the Narrow Sea."

- Stannis Baratheon

The Assault on Dragonstone was the last engagement of Robert's Rebellion, the civil war that had divided the Seven Kingdoms and had led to the fall of the Targaryen dynasty and the rise of the Baratheon dynasty.

Prelude
Following the Sack of King's Landing and the death of the Mad King, Robert Baratheon took the Iron Throne and was crowned Robert of the House Baratheon, First of His Name, King of the Andals and the First Men, Lord of the Seven Kingdoms and Protector of the Realm. However, the war was not over.

After Lord Mace Tyrell bent the knee and his siege of Storm's End was lifted, Robert sent his brother Stannis to Dragonstone, the island fortress located at the eponymous island, where Queen Rhaella and Prince Viserys had been sent following the rebel victory at the Battle of the Trident.

Events
Stannis led the Royal fleet aboard the Fury to capture Prince Viserys and Princess Daenerys, who had been recently born. However, before he arrived, Ser Willem Darry, one of the last Targaryen loyalists, smuggled the two children across the Narrow Sea.

Aftermath
Enraged over the escape of Viserys and Daenerys and blaming Stannis for it, Robert stripped Storm's End from him and granted it to their younger brother Renly. Instead, Robert granted Dragonstone to Stannis. Stannis took it as a slight, despite Dragonstone had been the traditional seat of the heir to the Seven Kingdoms.

In the books
In the A Song of Ice and Fire novels, the assault took place after the new royal fleet was built; almost a year after the end of the rebellion. The garrison of Dragonstone was ready to sell Viserys and Daenerys to to Stannis, but before they could go ahead Ser Willem Darry, along with four other loyalists, smuggled Viserys and Daenerys along with a wet nurse to the Braavosian Coastlands, were they would live in exile.