Khmer Empire
Relations with regional powers

During the formation of the empire, the Khmer had close cultural, political, and trade relations with Java and with the Srivijaya empire that lay beyond Khmer's southern seas. In 851 a Persian merchant named Sulaiman al-Tajir recorded an incident involving a Khmer king and a Maharaja of Zabaj. He described the story of a Khmer king who defied the power of Maharaja of Zabaj. It was said that the Javanese Sailendras staged a surprise attack on the Khmers by approaching the capital from the river. The young king was later punished by the Maharaja, and subsequently the kingdom became a vassal of the Sailendra dynasty.

Zabaj is the Arabic form of Javaka and might refer to Java or Srivijaya. The legend probably describes the predecessor or initial stage of the Khmer kingdom under Javanese dominion. The Legend of the Maharaja of Zabaj was later published by the historian Masoudi in his 947 book, "Meadows of Gold and Mines of Gems." The Kaladi inscription of Java (c. 909) mentioned Kmir (Khmer people or Cambodian) together with Campa (Champa) and Rman (Mon) as foreigners from mainland Southeast Asia who frequently came to Java to trade. The inscription suggests a maritime trade network had been established between Kambuja and Java (Mdang kingdom).

In 916 CE Arab historian Abu Zaid Hasan, recorded in a lengthy chronicle that the young, inexperienced king of Khmer, is hostile to Java. When the hostility becomes state policy and is known publicly, the King of Java attacked and captured the Khmer king. He was beheaded and the head brought to Java. The King of Java ordered the Minister of Khmer Empire to seek the successor. After being cleaned and embalmed, the head of the king was put in a vase and sent to the new Khmer king.

Throughout its history, the empire also was involved in series of wars and rivalries with the neighbouring kingdoms of Champa, Tambralinga, and Đại Việt – and later in its history with Siamese Sukhothai and Ayutthaya. The Khmer Empire's relations with its eastern neighbour Champa was exceptionally intense, as both sides struggled for domination in the region. The Cham fleet raided Angkor in 1177, and in 1203 the Khmer managed to push back and defeat Champa.

Arab writers of the 9th and 10th century hardly mention the region for anything other than its perceived backwardness, but they considered the king of Al-Hind (India and Southeast Asia) one of the four great kings in the world. The ruler of the Rashtrakuta Dynasty is described as the greatest king of Al-Hind, but even the lesser kings of Al-Hind including the kings of Java, Pagan Burma, and the Khmer kings of Cambodia are invariably depicted by the Arabs as extremely powerful and as being equipped with vast armies of men, horses, and often tens of thousands of elephants. They were also known to have been in possession of vast treasures of gold and silver. The Khmer rulers established relations with the Chola dynasty of South India.

The Khmer Empire seems to have maintained contact with Chinese dynasties; spanning from the late Tang period to the Yuan period. The relations with the Yuan dynasty was of great historical significance, since it produced The Customs of Cambodia (真臘風土記), an important insight into the Khmer Empire's daily life, culture and society. The report was written between 1296 and 1297 by the Yuan diplomat Zhou Daguan, sent by Temür Khan of Yuan dynasty to stay in Angkor.

Beginning in the 13th century, Khmer's relations with the Siamese were difficult and bitter, resulting in rivalry and hostility for centuries. In August 1296, Zhou Daguan recorded that in the recent war with the Siamese, the country was utterly devastated. This report confirmed that by the late 13th century, the Siamese warlords had revolted and disrupted the Khmer empire's hegemony, starting Siam's rise. By the 14th century, the Siamese Ayutthaya Kingdom became the Khmer empire's formidable rival, as Angkor was besieged and captured twice by Ayutthayan Siamese invaders in 1353 and 1394. Being the successor to the former Khmer dependency of Lavo (Lopburi), Ayutthaya also inherited traditions of Khmer prestige and statecraft. Unlike earlier Tai polities that had a relatively flat hierarchy, Ayutthaya adopted Angkor's more complex social stratification, the deification of kings (Devaraja) as well as Khmer honorifics and elaborate Brahmanic rituals. Specialist staff in early Ayutthaya, like scribes, court Brahmans, jurists, chamberlains, accountants, physicians and astrologers, usually came from the Khmer-speaking elites of city-states in the eastern Chaophraya Basin that had been under the influence of the Khmer Empire.

In the 1300s, the Lao prince Fa Ngum was exiled to lived in the royal court of Angkor. His father-in-law, the King of Cambodia, gave him a Khmer army to create a buffer state in what is now Laos. Fa Ngum conqured local principalities and established the Kingdom of Lan Xang. With the assistance of Khmer scholars, Fa Ngum introduced Theravada Buddhism and the culture of the Khmer Empire to the region.

A Javanese source, the Nagarakretagama canto 15, composed in 1365 in the Majapahit Empire, claimed Java had established diplomatic relations with Kambuja (Cambodia) together with Syangkayodhyapura (Ayutthaya), Dharmmanagari (Negara Sri Dharmaraja), Rajapura (Ratchaburi) and Singhanagari (Songkla), Marutma (Martaban or Mottama, Southern Myanmar), Champa and Yawana (Annam). This record describes the political situations in Mainland Southeast Asia in the mid-14th century; although the Cambodian kingdom still survived, the rise of Siamese Ayutthaya had taken its toll. Finally, the empire fell, marked by the abandonment of Angkor for Phnom Penh in 1431, caused by Siamese pressure.
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