Rama IV, or Mongkut as he is often called in English, was the fourth king of the currently reigning Chakri dynasty in what is now known as Thailand, then Siam. He was called to the throne in 1851, when his elder half brother, Rama III, died; his brother had inherited the throne when their father, Rama II, passed away. Rama IV himself died in 1868; power passed to his son, Chulalongkorn or Rama V.
Rama IV inherited his power relatively late in life; he was 47 years old when he was crowned, and had been a Theravada Buddhist monk for 27 years. He had used his years in the monastery to good purpose: he had learned English, French, and Latin from farang missionaries; he had studied Pali and excelled in Buddhist studies; and had gained a good knowledge of Thai and western science and technology. As a monk he had also had a chance to see something of the life of ordinary Thai people, travelling to wat in remote areas of the kingdom where he could preach and learn from commoners in a way he could not as a prince in the capital.
Rama IV appears to have had a volatile temper, and could become quite angry when he thought he was judged too harshly. Reverend Dan Bradley, an American missionary who published a newspaper in Bangkok during Rama IV's tenure, sometimes criticized the king for some action or non-action, and the monarch responded with sharply worded letters or even, on more than one occasion, by shutting the paper down. Rama IV was known to be rather jealous of his uparat, who many farang found easier to deal with than the king. In spite of these all-too-human failings, Mongkut is generally thought to have been a wise ruler who loved his subjects and worked hard to improve their lives. He welcomed innovations that he thought were good for his country, and unlike the xenophobic brother who ruled before him, welcomed farang and sought to learn as much as he could from them. His ability to converse with them in their own languages - his English, though idiosyncratic, was quite comprehensible - was an aid to him in this.
When he became king, one of Rama IV's responsibilites was to father heirs to lead the kingdom after his death, a duty he fulfilled with alacrity. He took numerous wives and concubines who lived in a section of the Grand Palace with him, in an inner sanctum which amounted to a small city of women complete with women shopkeepers, crafts workers, and police. The women were traditionally forbidden to see any man but the king, though Rama IV broke with tradition when he brought them with him on an expedition to southern Siam to view an eclipse and introduced them to his guests. He allowed his wives who had not borne him children to leave the palace if they wished, and also decreed that girls could not be forced by their parents to marry against their will.
You may think as you read this that all this is interesting in a distant sort of way, but describing this king in detail is important, because you may know him as the petulant screamer portrayed by Yul Brynner in the movie The King and I. Thai people are deeply offended by this insulting portrayal of their beloved king, and the movie, and its successors, is banned in the kingdom. The impetus behind these characterizations of Rama IV was the writings of one Anna Leonowens, the "I" in The King and I, who spent five years in Siam as English teacher to the royal children. I have lots to say about her in my node on her, if you're interested.
One of the many aspects of the movie which so many Thai find offensive is the suggestion that Rama IV was in love with Anna. The story is plausible: though he had many women at his disposal, all were probably of Asian descent, and there are indications that the king was interested in obtaining a farang addition to his harem. Anna wrote that Mongkut offered "enormous sums...year after year...for an English woman of beauty and parentage to crown the sensational collection" of women he enjoyed, though to no avail. It is doubtful that this quest was quite as constant as Anna suggested; however, in a letter from the king to the head of his 1857 embassy to Queen Victoria, Rama IV wrote:
My only regret is that, having had the good fortune to behold the beauty of angels, you have to return home empty-handed, for you cannot buy them and bring them back like Chinese women. Nevertheless, it might be a good idea to buy some of their costumes and bring them back home to dress up some of our earthly beauties here for the sake of variety.
According to the London Times, the embassy visited Kate Hamilton's famous London brothel, "where one of the senior envoys...continued to dip liberally into the treasures of the two kingdoms until daybreak". The press also reported that the first ambassador offered a woman who caught his eye £3000 to become his fifty-ninth wife.
All this is true, but it's also true that Anna never even intimated that the king was interested in her; the romance is a Hollywood fiction introduced to make the story more commercially viable. In the western world, Rama IV may be forever confused with a strutting Yul Brynner who loved a governess, but in Thailand he is known as the father of the modern Thai nation-state, venerated to this day.
There's lots more information at my nodes on Mongkut, Chulalongkorn, and Anna Leonowens.
I've quoted from Anna Leonowens' The English Governess at the Siamese Court, originally published in 1870. The extract from Rama V's letter to his ambassador is cited in Abbot Low Moffat's biography of Rama V, Mongkut, the King of Siam, while the information from the London Times is cited in John Blofeld's biography of him, King Maha Mongkut of Siam.