In 1880, the French colonial administration to provide subject status to Vietnamese residents in Cambodia. Over the next fifty years, large numbers of Vietnamese migrated to Cambodia. Population censuses conducted by the French recorded an increase in the Vietnamese population from about 4,500 in the 1860s to almost 200,000 at the end of the 1930s. When the Japanese invaded Indochina in 1940, Vietnamese nationalists in Cambodia launched a brief but unsuccessful attempt to attack the French colonial administrators. In 1954, a citizenship law was passed on the basis of knowledge in the Khmer language and national origin, and effectively excluded most Vietnamese and Chinese Cambodians. At the grassroots level, Vietnamese also faced occasional cases of violent intimidation from the Cambodians. During a Sangkum congress in 1962, politicians debated on the issue of citizenship on Cambodia's ethnic minorities and a resolution was passed not to grant naturalization of Vietnamese residents.