In the 1020s Denmark was threatened by Norway and Sweden, and in 1026 Cnut decided to strengthen its defences by bringing over his eight-year-old son to be the future king under a council headed by his brother-in-law, Earl Ulf. However, Ulf alienated Cnut by getting the Danish provinces to acknowledge Harthacnut as king without reference to Cnut's overall authority and by failing to take vigorous measures to meet Norwegian and Swedish invasions, instead waiting for Cnut's assistance. In 1027, Cnut arrived with a fleet. He forgave Harthacnut his insubordination in view of his youth but had Ulf murdered. He drove the invaders out of Denmark and established his authority over Norway, returning to England in 1028 and leaving Denmark to be ruled by King Harthacnut.