The War of the Split Stag began in the year 442 AC and ended in the year 444 AC. It arose due to the dispute between two Baratheon twins, Lyonel Baratheon and Durran Baratheon, who both believed themselves to be the rightful heirs to the Iron Throne.
Lyonel Baratheon, the elder of the two twins, was chosen by a Grand Council to become King after the death of Orys Baratheon and the subsequent execution of the Treacherous Twelve involved in his murder. Shortly after his coronation, Durran fled King's Landing and began rallying support to his cause.
Durran and his supporters brought war to Lyonel and his loyalists, eventually culminating in a siege of King's Landing where Lyonel was killed.
Durran Baratheon sat the throne for a single day before the Faith Militant - who supported Lyonel Baratheon - overwhelmed his soldiers and had him executed.