 He later attended the Cavalry Officers' Candidate School at Trandum, followed by enrollment at the Norwegian Military Academy, from which he graduated in 1959.On 21 September 1957 at the death of his grandfather, Harald became crown prince at the age of 20 and he attended the Council of State for the first time six days later and took the oath to the Constitution of Norway on 21 February 1958.In the same year, he also served as regent in the King's absence for the first time.