He became Chief Councillor to King Edward I, son and successor of Henry III. While the king was engaged in military conflicts with the Scots, Henry was appointed Protector of the Realm. Having taken part in the Conquest of Wales in 1282, Henry was granted the Lordship of Denbigh and built Denbigh Castle. In 1296 he went to France with Edmund, Earl of Lancaster ("Edmund Crouchback"), the king's younger brother, on whose death in that year he succeeded as commander of the English forces in Gascony; he returned to England early in 1298. He was at the Battle of Falkirk in 1298 and at the Siege of Carlaverock Castle in 1300, both in Scotland. The Roll of Carlaverock records his coat of arms in verse as: "Or, a lion rampant purpure." In November 1300 he was sent on a mission to Rome to complain to the Pope about injury done by the Scots. He was present at King Edward's death in July 1307. For a short time he was friendly with the new king Edward II, and with his favourite Piers Gaveston, but quickly changed his loyalties and joined Thomas, 2nd Earl of Lancaster and the baronial party. He was one of the "Ordainers" appointed in 1311 and was Regent of the Kingdom during the king's absence in Scotland in the same year. He transferred Stanlow Abbey, the Cistercian monastery of which his family was patron, to Whalley Abbey.