The King and I is the fifth musical by the team of composer Richard Rodgers and dramatist Oscar Hammerstein II. It is based on Margaret Landon's novel, Anna and the King of Siam (1944), which is in turn derived from the memoirs of Anna Leonowens, governess to the children of King Mongkut of Siam in the early 1860s. The musical's plot relates the experiences of Anna, a British schoolteacher hired as part of the King's drive to modernize his country. The relationship between the King and Anna is marked by conflict through much of the piece, as well as by a love to which neither can admit.
Historical background
Mongkut, King of Siam, was about 57 years old in 1861. He had lived half his life as a Buddhist monk, was an able scholar, and founded a new order of Buddhism and a temple in Bangkok (paid for by his half-brother, King Nangklao). Through his decades of devotion, Mongkut acquired an ascetic lifestyle and a firm grasp of Western languages. When Nangklao died in 1850, Mongkut became king. At that time, various European countries were striving for dominance, and American traders sought greater influence in Southeast Asia. He ultimately succeeded in keeping Siam an independent nation, partly by familiarizing his heirs and harem with Western ways.  In 1861, Mongkut wrote to his Singapore agent, Tan Kim Ching, asking him to find a British lady to be governess to the royal children. At the time, the British community in Singapore was small, and the choice fell on a recent arrival there, Anna Leonowens (1831-1915), who was running a small nursery school in the colony. Leonowens was the Anglo-Indian daughter of an Indian Army soldier and the widow of Thomas Owens, a clerk and hotel keeper. She had arrived in Singapore two years previously, claiming to be the genteel widow of an officer and explaining her dark complexion by stating that she was Welsh by birth. Her deception was not detected until long after her death, and had still not come to light when The King and I was written.  Upon receiving the King's invitation, Leonowens sent her daughter, Avis, to school in England, to give Avis the social advantage of a prestigious British education, and traveled to Bangkok with her five-year-old son, Louis. King Mongkut had sought a Briton to teach his children and wives after trying local missionaries, who used the opportunity to proselytize. Leonowens initially asked for $150 in Singapore currency per month. Her additional request, to live in or near the missionary community to ensure she was not deprived of Western company, aroused suspicion in Mongkut, who cautioned in a letter, "we need not have teacher of Christianity as they are abundant here". King Mongkut and Leonowens came to an agreement: $100 per month and a residence near the royal palace. At a time when most transport in Bangkok was by boat, Mongkut did not wish to have to arrange for the teacher to get to work every day. Leonowens and Louis temporarily lived as guests of Mongkut's prime minister, and after the first house offered was found to be unsuitable, the family moved into a brick residence (wooden structures decayed quickly in Bangkok's climate) within walking distance of the palace.  In 1867, Leonowens took a six-month leave of absence to visit her daughter Avis in England, intending to deposit Louis at a school in Ireland and return to Siam with Avis. However, due to unexpected delays and opportunities for further travel, Leonowens was still abroad in late 1868, when Mongkut fell ill and died. Leonowens did not return to Siam, although she continued to correspond with her former pupil, the new king Chulalongkorn.

Q: whats the historical background of the king
A: Mongkut, King of Siam, was about 57 years old in 1861. He had lived half his life as a Buddhist monk,
when did he become king
When Nangklao died in 1850, Mongkut became king.
When Nangklao died in 1850, Mongkut became king.