Lý Huệ Tông
Having been mentally ill for a long time, the Emperor Lý Huệ Tông ultimately decided to cede the throne of the Lý Dynasty to crown princess Lý Chiêu Hoàng in October of Lunar calendar, 1224. Lý Chiêu Hoàng thus became the only empress regnant in history of Vietnam. This decision of Lý Huệ Tông was considered by the historian Ngô Sĩ Liên as the crucial factor leading to the collapse of the Lý Dynasty, because if he had chosen a capable member of the royal family instead of his young princess, the situation for the Dynasty could have been different.