Battle of Summerhall

The Battle of Summerhall was the first battle of Robert's Rebellion. It was actually three battles in a single day, all three won by Robert Baratheon, then Lord of Storm's End. Afterward, Robert marched on Ashford in the Reach, where he would experience his only defeat, leaving his brother, Stannis Baratheon, to hold Storm's End.

Season 1
King Robert Baratheon recalls how he made his first kill during the battle of Summerhall.

In the books
In the A Song of Ice and Fire books, the Battle of Summerhall was actually three battles all fought on one day. Robert hit three independent armies of Targaryen-loyalist Stormlords before they could converge upon him. Lords Grandison, Cafferen, and Fell planned to join their hosts at Summerhall and march on Storm's End, but Robert learned of their plans and struck first, attacking each army individually. Lord Fell died in the battle and Robert's forces captured his son, Silveraxe. Lords Grandison and Cafferen, both loyalist commanders, went over to Robert after the battle, as did Silveraxe.