King Henry III of England reigned from 1216 to 1272, making him one of the longest-serving monarchs in English history. He ascended to the throne at just nine years old after the death of his father, King John. His reign was marked by a struggle for power between the monarchy and the barons, leading to significant conflicts, including the Barons' War. Henry III is known for his efforts to strengthen the monarchy and improve the administration of justice. He also initiated the construction of Westminster Abbey, which became a significant symbol of English royalty. His reign laid the groundwork for future developments in the English government.