The Battle of the Horn was a major conflict that occurred during the War of the Lords Scorned in 333 AC, and served as the first major battle between rebel and royal forces.
The Lords Scorned, emboldened by early results, pressed forward against the Crown's forces, while Aegon himself knew he could not afford to suffer losses in the field. Before the battle got out of control, the King sounded the retreat, paving way for a rebel victory.
Aegon's retreat gave room for him to focus on a more attrition-based defence of the Crownlands, which ultimately helped shift the balance and tone of the war a few months later.