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.