The region where Pol Pot and the others moved to was inhabited by tribal minorities, the Khmer Loeu, whose rough treatment (including resettlement and forced assimilation) at the hands of the central government made them willing recruits for a guerrilla struggle. In 1965, Pol Pot made a visit of several months to North Vietnam and China. From the 1950s on, Pol Pot had made frequent visits to the People's Republic of China, receiving political and military training—especially on the theory of "dictatorship of the proletariat"—from the personnel of the CPC. From November 1965 to February 1966, Pol Pot received training from high-ranking CPC officials such as Chen Boda and Zhang Chunqiao, on topics such as the communist revolution in China, class conflicts, and Communist International. Pol Pot was particularly impressed by the lecture on political purge by Kang Sheng. This experience had enhanced his prestige when he returned to the WPK's "liberated areas". Despite friendly relations between Sihanouk and the Chinese, the latter kept Pol Pot's visit a secret from Sihanouk.