History of Cambodia
The name Siem Reap can be translated to mean 'Defeat of Siam', and it refers to the centuries-old conflict between the Siamese and Khmer peoples. Under the rule of the Ayutthaya Kingdom in the 18th century. it was referred to as Nakhom Siam. According to oral tradition, the name was given by king Ang Chan (1516-1566) as "Siem Reap", meaning "the flat defeat of Siam" after he defeated the Thai King Maha Chakkrophot who send an arm to attack Cambodia in 1549. Scholars however consider this a modern falk etymology and that the actual origin of the name of Siem Reap is Unknown.
The story was told that King Ang Chan of Cambodia tried to assert further independence from Siam. The Siamese also had been through internal trouble themselves during these years. King Chairacha was poisoned by his concubine, Lady Sri Sudachan, who committed adultery with a commoner, Worawongsathirat, while he was on a campaign against Chiang Mai. Sudachan then raised Worawongsathirat to the throne. The nobles hated Worawongsathirat and lured the usurper and his family to a place outside the city where he was assassinated, together with Sudachan and a new-born daughter, during the royal family's procession by barge to see a white elephant (allegedly just captured). The nobles then invited Prince Thianracha, who was a monk in a monastery, to give up that role and ascend the throne under the title of King Maha Chakkraphat (1543-1569). Being informed of the internal troubles in Ayutthaya King Ang Chan attacked Prachin Buri in 1549 and successfully took away its Siamese inhabitants. There he obtained information that of Maha Chakkraphat's coronation, signaling that the question of succession in Ayutthaya has thus been settled. Ang Chan therefore retreated and did not advance and further. King Maha Chakkraphat was very angry at this, but his hands were tied, because the Burmese had just come by way of the Three Pagodas Pass; they took Kanchanaburi, Suphanburi, and appeared in front of Ayutthaya.
Because King Ang Chan refused to give King Maha Chakkraphat a white elephant when he asked for it, it is indicated that Ang Chan declined any symbol of vassalage to Siam Maha Chakkraphat's attention was now turned towards Cambodia. He put Prince Ong, the governor of Sawankhalok and Srey's son, in charge of an expedition against Cambodia. Ang Chan counter-attacked, and shot Prince Ong dead on an elephant's back, and his army routed the Siamese and captured no less than 10, 000 Siamese troops. It was because of this victory over Siaamese the that King Ang Chan renamed the battleground as "Siem Reap" meaning "the flat defeat of Siam".
However, most sources indicate the decline of Angkor more than a century prior, when an Ayutthaya military expedition captured and sacked Angkor Wat in 1431, initiating a period of vassal rule there. This event coincided with the decline of the city, though there is not a full understanding of the reasons behind the abandonment of Angkor Wat, which may have included changes in the environment and failings of infrastructure.
From the sixteenth through to the nineteenth centuries, the feuds among the Khmer lords caused the interventions and domination from their more powerful neighbors: Vietnam and Siam, Siem Reap, along with Battambang (Phra Tabong) and Sisophon, major cities in the north western part of Cambodia, were under Siamese administration known as Inner Cambodia from 1795 till 1907 when the province was ceded to French Indochina.