The Khmer Empire ruled mainland Southeast Asia from 802 to 1431 AD from a string of capitals north of the Tonle Sap lake in what is now Cambodia, the best known being Angkor. At its widest, around 1290 by the political scientist Rein Taagepera’s estimate, it covered about 1 million km², taking in most of today’s Cambodia, much of northeastern and central Thailand, southern Laos and, after 1203, much of Champa in central Vietnam. It was ruled by Hindu and later Buddhist god-kings, from Jayavarman II to Ponhea Yat, and its wealth rested on rice grown with the help of huge reservoirs. That system made Angkor the largest pre-industrial city in the world by area, sprawling over at least 1,000 km² with perhaps 700,000 to 900,000 people at its 13th-century peak. The empire called itself Kambuja, the root of the modern name Kampuchea.
Jayavarman II on Phnom Kulen, 802: how a Khmer state took shape
Before 802 the lower Mekong was divided among rival principalities, the successors of the earlier state known as Chenla. One of their princes, Jayavarman II, began his career around 781 at Indrapura, probably the site now called Banteay Prey Nokor near Kampong Cham, beside the Mekong in eastern Cambodia. Over the next two decades he defeated competing kings and moved his court steadily northwest, towards the great lake.
The founding moment came in 802 on Phnom Kulen, a sandstone plateau north of the later Angkor site, then called Mahendraparvata. There Jayavarman II held a consecration ritual drawn from Hindu tradition and proclaimed himself chakravartin, a universal ruler, and devaraja, a god-king. He also declared independence from a place the inscriptions call “Java”. Historians long argued over whether that meant the Indonesian island, Champa or somewhere else; in 2013 the epigraphist Arlo Griffiths argued convincingly that it meant the island of Java.
Two things in that story explain the later geography. First, the court moved off the Mekong and into the plain between the Kulen hills and the Tonle Sap, where rivers ran down from the plateau towards the lake. Second, kingship was tied to temple-building: every strong ruler would mark his reign with a new state temple, often a stepped “temple-mountain” representing Mount Meru. Jayavarman II settled at Hariharalaya, near the modern town of Roluos, and died in 835. A later king, Indravarman I (r. 877–889), developed Hariharalaya further with the Preah Ko temple, the Bakong around 881 and large irrigation works.
Water as wealth: the Tonle Sap, the barays and a hydraulic city
The empire grew because its heartland produced a food surplus few rivals could match. The Tonle Sap is the largest freshwater lake in Southeast Asia and it breathes with the monsoon. In the dry season it shrinks to about 2,500 to 3,000 km² and around a metre deep; in the rainy season the Mekong rises so high that the Tonle Sap River reverses direction and pushes water back into the lake, which swells to about 16,000 km² and 9 to 14 metres deep. When the Mekong falls again, the river flips and drains the lake. The receding flood leaves fertile silt on a broad plain and one of the richest freshwater fisheries in the world.
Khmer kings built on that natural rhythm. Fish, eaten fresh or as the fermented paste prahok, supplied protein; rice supplied everything else. To spread water through the dry months they dug canals, moats and enormous rectangular reservoirs called barays. Yasovarman I (r. 889–915) built the East Baray, about 7.1 to 7.2 km long and 1.7 km wide, when he founded Yasodharapura, the first city on the Angkor site, around the hill of Phnom Bakheng. In the 11th century Suryavarman I and Udayadityavarman II added the West Baray, about 7.8 by 2.1 km, held in by earth dikes nearly 12 metres high. It still holds up to 53 million cubic metres of water. Whether the barays mainly irrigated fields or served as symbols of the cosmic ocean around Mount Meru is still argued, and they probably did both.
In 2007 an international team using satellite images and radar showed how far this network reached. Roads, canals and embankments tied an area of at least 1,000 km² to the temple core, making Angkor the largest urban complex anywhere before the Industrial Revolution, well ahead of the Maya city of Tikal. Most people lived in low-density villages of stilt houses among the rice fields rather than inside walls, which is why the city’s edges are so hard to draw. At its height the system allowed up to three rice harvests a year. Zhou Daguan, a Chinese envoy who stayed in 1296–1297, recorded that Angkor’s open-air market was run mainly by women.
From Yasodharapura to Angkor Thom, 889–1219: capitals, conquests and the empire’s peak
Royal power moved around the plain more than once. Early in the 10th century Jayavarman IV shifted the court about 100 km northeast to Koh Ker, then called Lingapura, until Rajendravarman II (r. 944–968) brought it back to Yasodharapura and built Pre Rup and the East Mebon. His son Jayavarman V (r. 968–1001) presided over a cultural flowering that produced Banteay Srei, consecrated in 967 by royal counsellors and prized for its fine carving, and Ta Keo, the first Angkor temple built entirely of sandstone.
Expansion came through war and diplomacy. Suryavarman I (r. 1006–1050) seized Angkor after a decade of civil conflict, survived repeated revolts and allied with the Chola emperor Rajendra I of south India against Tambralinga, a kingdom on the Malay Peninsula. Suryavarman II (r. 1113–1150) reunited the realm and spent about 37 years building Angkor Wat, dedicated to Vishnu and set on a site of 162.6 hectares, the largest religious complex in the world. He sacked Vijaya, the capital of Champa in central Vietnam, in 1145, though the Khmer were driven out in 1149.
The low point came in 1177, when a Cham fleet under Jaya Indravarman IV sailed up the Tonle Sap, raided the capital and killed King Tribhuvanadityavarman. The recovery made the empire’s peak. Jayavarman VII (r. 1181–1219), a Mahayana Buddhist, retook Angkor, fought Champa for 22 more years and conquered much of its territory in 1203. Chinese sources say he also added Pegu, in what is now Myanmar, in 1195. He built a new walled capital, Angkor Thom, around the Bayon, whose towers carry giant carved faces read variously as the bodhisattva Avalokiteshvara or the king himself, plus Ta Prohm for his mother and Preah Khan for his father.
The empire at full stretch: roads and rest houses from Phimai to Champa
What held a realm of around 1 million km² together was a network of raised roads radiating from Angkor. The Preah Khan inscription of 1191 counts 121 rest houses with hearths along the royal roads: 57 on the routes to Champa in the east and 17 on the road northwest to Vimayapura, today’s Phimai in Thailand’s Nakhon Ratchasima province. Jayavarman VII is also credited with 102 hospitals across the empire; of the seven inscriptions that record their founding in 1186, five come from the Nakhon Ratchasima region around Phimai.
Phimai shows how the provinces worked. Its temple, Prasat Hin Phimai, faces Angkor rather than east, and its enclosure of about 1,020 by 580 metres rivals Angkor Wat’s. Small villages fed regional centres like it, which in turn sent goods to the capital in exchange for pottery and Chinese trade goods. Further west, Khmer inscriptions from Sa Kaeo and Prachin Buri suggest districts administered directly from Angkor, and Lavo, later Lopburi, was a Khmer dependency in the Chao Phraya valley. To the north, temples such as Vat Phou in southern Laos marked Khmer religious reach along the Mekong.
Why Angkor faded, 1296–1431: Siam, drought and a new religion
Angkor’s decline took more than a century, and historians still disagree over which cause weighed most. The first was military. When Zhou Daguan arrived in August 1296 he reported that a recent war with the Siamese had left the country “utterly devastated”. In 1351 the Ayutthaya Kingdom was founded on the lower Chao Phraya, on land long under Khmer influence. Its armies besieged Angkor in 1352 and took it the next year; a Khmer king regained the throne in 1357, but Ayutthaya took the city again in 1394.
The second cause was the water system itself. Researchers have identified a period of strong monsoons followed by severe drought in the 14th and 15th centuries. Forest clearance on the Kulen hills for new rice fields sent silt down into the canals, and floods broke infrastructure that a weakened court could no longer repair. Since the whole city depended on that network, damage in one place spread through the rest.
The third was a change of belief. Srindravarman, who took power in 1295, followed Theravada Buddhism, which had spread from Sri Lanka. The last Sanskrit inscription is dated 1327. Kings who were no longer god-kings had less reason to raise huge state temples and, many historians argue, less power to call out the labour that maintained canals and dikes. Disputed successions added to the strain: of 27 Angkorian rulers, 11 lacked a clear claim to the throne. Meanwhile trade was shifting to the sea, where Angkor, far inland, was poorly placed.
After 1431: Phnom Penh, Angkor’s afterlife and UNESCO in 1992
The date usually given for the end is 1431, when King Ponhea Yat abandoned Angkor as indefensible and moved south to the Phnom Penh area, where the Tonle Sap meets the Mekong. Later courts sat at Longvek and Oudong. The logic was geographic: a river junction could tap the growing maritime trade, while a flood plain at the far end of the lake could not. Angkor was never wholly empty. Barom Reachea I (r. 1566–1576) briefly returned the court there, Angkor Wat became a Buddhist monastery that is still in use, and a Japanese visitor, Ukondayu Kazufusa, recorded celebrating the Khmer New Year at Angkor in 1632.
The empire’s mark on the region is wider than its ruins. Ayutthaya, the power that broke Angkor, borrowed its court ritual, honorifics and idea of the divine king, often through Khmer-speaking officials. Angkor became a UNESCO World Heritage Site in 1992 and now draws close to two million visitors a year to Siem Reap province. The temple that put the empire on Cambodia’s flag is covered in more depth in our guide to Angkor Wat. The lake that fed it all is described in our Tonle Sap geography guide, and the wider landscape of plains, the Cardamom Mountains and the Mekong is in our overview of Cambodia’s physical geography.