The Khmer Empire flourished in the area from the 9th to the 13th century. In the 13th century, Theravada Buddhism was introduced through monks from Sri Lanka. This eventually became a popular religion in the country. The Khmer Empire centre of power was Angkor. Thailand took control of the region after a series of wars. In 1863, King Norodom sought protection of France from the Thailand and Vietnam. Cambodia continued as a protectorate of France from 1863 to 1953. Under the reign of King Norodom Sihanouk, Cambodia gained independence from France on November 9, 1953. It became a constitutional monarchy under King Norodom Sihanouk.
In November 1978, Vietnam invaded Cambodia to stop Khmer Rouge incursions across the border and the genocide in Cambodia. Peace efforts began in Paris in 1989. This resulted in a comprehensive peace settlement. The United Nations was given a mandate to enforce a ceasefire, and deal with refugees and disarmament.