
The Khmer Empire (Khmer: ចក្រភពខ្មែរ), or the Angkorian Empire (Khmer: ចក្រភពអង្គរ), are the terms that historians use to refer to Cambodia from the 9th to the 15th century, when the nation was a Hindu-Buddhist empire in Southeast Asia. The empire grew out of the former civilizations of Funan and Chenla, which at times ruled over and/or vassalised most of mainland Southeast Asia and parts of Southern China, stretching from the tip of the Indochinese Peninsula northward to the modern Yunnan province of China, and from Vietnam westward to Myanmar. At its peak, the Khmer Empire was larger than the Byzantine Empire (Eastern Roman Empire), which existed around the same time.
Perhaps its most notable legacy is the site of Angkor, in present-day Cambodia, the Khmer capital during the empire's zenith. The majestic monuments of Angkor, such as Angkor Wat and Bayon, bear testimony to the Khmer Empire's immense power and wealth, impressive art and culture, architectural technique, aesthetic achievements, and the variety of belief systems that it patronised over time. Satellite imaging has revealed that Angkor, during its peak in the 11th to 13th centuries, was the largest pre-industrial urban centre in the world.
The beginning of the era of the Khmer Empire is conventionally dated to 802, when King Jayavarman II declared himself chakravartin ("universal ruler", title equivalent to "emperor") in the Phnom Kulen mountains. The empire's end is marked with the Siege of Angkor by the Siamese Ayutthaya Kingdom, in 1431. The reason the Khmer Empire collapsed is considered a mystery. Researchers have determined that a period of strong monsoon rains was succeeded by a drought in the region, causing damage to infrastructure. Variability between droughts and flooding was also a problem. The collapse of the water system may have caused people to abandon the city of Angkor.