The Treaty of Alton was an agreement signed in 1101 between Henry Beauclerc and his older brother Robert Curthose in which Robert agreed to recognize Henry as the king of England in exchange for a yearly stipend and other concessions.
Robert's absence in the First Crusade at the time of William's death, as well as his poor reputation among the Anglo-Norman barons, allowed the youngest brother, Henry Beauclerc, to claim the throne with popular support.
Encouraged by his advisor Ranulf Flambard, he invaded the Kingdom of England in order to claim the throne.
In the Treaty of Alton, Robert agreed to renounce his claim to the English throne in exchange for a yearly stipend of 3,000 marks and the concession of all but one of Henry's possessions in the Duchy of Normandy.
In 1105, Henry invaded Normandy and defeated his brother's army the following year at the Battle of Tinchebray.