Notable anti-Khmer sentiment
Historically, Southern part of Vietnam before was not under Vietnamese rule but under Khmer Empire's subject, however, following the Vietnamese invasion to the South, the gate to the South China Sea has eventually closed for Cambodia. During the time of Minh Mạng of Nguyễn Dynasty, Vietnam implied a policy as "Hán di hữu ngạn" 漢夷有限 ("the Vietnamese and the barbarians must have clear borders") when differentiating between Khmers and the Vietnamese. Emperor Minh Mạng, the son of Gia Long stated with regards to the Vietnamese forcing the ethnic minorities to follow Sino-Vietnamese customs that "We must hope that their barbarian habits will be subconsciously dissipated, and that they will daily become more infected by Han [Sino-Vietnamese] customs."
There were numerous Vietnamese invasions and occupation of Cambodia, although it was rather short-lived.
After the Vietnamese invasion of Cambodia later, at 1979, overthrew the Khmer Rouge, Vietnamese troops stationed to Cambodia until 1989 to stabilize and partially occupy the country, helping Cambodians 'recover' from war, as well as controlling the Cambodian ruling elites.
Currently, with the recent anti-Vietnamese and pro-Chinese movement in Cambodia launched by Sam Rainsy and Cambodia's support for China in the South China Sea's conflict, Vietnamese have become more hostile towards Cambodia and Cambodian people, seeing Cambodians as ungrateful and traitors who do not remember the past.
Hatreds against Khmers began with the rise of Ayutthaya Kingdom. The Siamese had ransacked Angkor Wat and invaded Cambodia many times in the history. Notably, Siamese Prince and King Naresuan had launched a brutal invasion which ransacked entire of Cambodia and Khmers being forced to Siam as its slave labors. It became a cure which would be remembered as Siamese atrocities against Khmers.
In the later times, when Vietnam rose to become a fierce rival, Siam had fought against Vietnam. However, many of Siam's attempt mostly crossed through Cambodia, which they had raped, looted and enslaved many Khmers during its road of invading Vietnam, thus led to many hostilities and hatreds against Cambodians among Siamese.
Since then, a numerous of disputes continued. In January 2003, riots broke out in Phnom Penh after a Cambodian newspaper incorrectly reported that a Thai actress had stated Angkor Wat properly belonged to Thailand. On 29 January, the Thai embassy was burned, and hundreds of Thai immigrants fled the country to avoid the violence. Cambodians in Phnom Penh burned photos of King Bhumibol Adulyadej and Thais in Bangkok protested in front of the Cambodian embassy, burning Cambodian flags. This eventually led to the Thai government to sever diplomatic ties with Cambodia. Prime Minister Hun Sen banned Thai shows and films on TV stations. Throughout 2008-13, Thai and Cambodian military forces did skirmish on each other over the Preah Vihear, leading to the Cambodian–Thai border dispute. The International Court of Justice's decision in the dispute favored Cambodia, which sparked anger among Thai citizens.
Many Thais also see Cambodians as poor and uneducated, and there are even hatreds against Khmer immigrants.
The Philippines, which also has disputes with China, has been the major critics who criticized Cambodia for backing China. This led to some Filipinos believe Cambodia is a traitor of ASEAN and a puppet of China.