Complex sentence: Ferdinand I entered into negotiations with James to prevent open warfare, but the negotiations failed, so Ferdinand I occupied castles and strong points in Urgell, and Ferdinand officially dissolved the County of Urgell in 1413 and the area came under the Count of Barcelona.
Split into simpler sentences: Ferdinand I entered unsuccessfully into negotiations with James to prevent open warfare and then occupied castles and strong points in Urgell. He officially dissolved the County of Urgell in 1413 and the area came under the Count of Barcelona.