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.