Following the still unexplained shooting to death of King Ananda on 9 June 1946, that task fell to the 18 year-old Bhumibol.
Thailand was divided then between progressive politicians and ambitious military men who preferred a weaker monarchy, or perhaps no monarchy at all, and members of the royal aristocracy who were determined to rebuild a political system with the monarchy at its heart.
The royalists relied on the young Bhumibol for this plan, and over the next 40 years it succeeded.
Royal roles
Until the mid 1950s the king's position was still too insecure for him to challenge the then-military strongman Phibul Songkram, who had also ruled during the war. He was not even allowed to travel freely outside Bangkok.