The book's theme is most certainly equality. As the Civil War rages in America, Anna Leonowens, a British widow, travels to Siam (Thailand) to teach English as a language and British customs to the king's children.
Because the author, Margaret Landon, portrays the king as a forward-thinking individual who cares enough about "modernizing" his family and court so that they can be recognized in the world, she is able to stress the strange dichotomy between that way of thinking and the king's initial refusal to latch on to the civilized idea of equality for everyone in his country.
In the novel, Anna and the king discuss Abraham Lincoln and Lincoln's goal to bring equality to America and England's earlier abolition of slavery.
In the novel, the king eventually comes to realize that he cannot fully promote Western ideas if he wants to hold onto certain unequal aspects of his culture (owning slaves, etc.).
While the truth in the novel which is based on two memoirs by the real Anna has often been brought into question, it is interesting that all versions of the story discuss the idea of equality and that shortly after the king (who is really King Mongkut) dies, his prime minister frees all Siamese slaves and prohibits visitors from falling prostrate in front of the king. So, even in the real-life accounts of the "King of Siam," the idea of equality is significant.
Anna and The King of Siam was published in 1944, near the end of the period of European colonialism, but set in the 1860s, the height of European imperialism. While a work of fiction, it is based on two memoirs published in the 1870s by Anna Leonowens, who spent time as a tutor to King Mongkut's wives and children.
The theme of the book is that by adopting the right morality, such as that of an English woman of character, a society can change and progress.
The novel is at pains to show Anna as a woman of strength and moral virtue. In its opening pages, as Anna and her son approach Bangkok, the captain of the steamer on which they travel advises her that what she is doing is "a man's job" and warns her that "people go in there and never come out again."
Anna from the start expresses her unyielding moral values: "I can't go back now. I've given my word." Symbolizing both European civilization and her own moral fierceness, she wears a brooch "into which were set two tiger claws."
Two very important values she brings from the west are freedom and equality, which she believes in unwaveringly and transmits to the king's son and heir. The young prince Chulalongkorn learns from Anna about the United States freeing the slaves, and when he becomes king, he frees the Siamese slaves and institutes progressive reforms.
The book critiques western imperialism because it contradicts the deeper values of freedom and equality that Anna hopes to convey as all-important. We learn, for instance, that when the French take over Cambodia, they give themselves special privileges, such as release from import and export duties. The book values Siam as the last free state in Asia and implies that it can keep its autonomy from imperialist tyranny by adopting the freedom and equality of the western world.