In April 1975, the Khmer Rouge seized power in Cambodia, and in January 1976, Democratic Kampuchea was established. During the Cambodian genocide, the CCP was the main international patron of the Khmer Rouge, supplying "more than 15,000 military advisers" and most of its external aid.[82] It is estimated that at least 90% of the foreign aid to Khmer Rouge came from China, with 1975 alone seeing US$1 billion in interest-free economic and military aid and US$20 million gift, which was "the biggest aid ever given to any one country by China"
And if you read the article, north vietnam was their main backer before.
