Union discontinued aid to North Korea. China was meanwhile unwilling to increase its aid, and, as a result, several industries in North Korea were on the brink of disaster. Mao Zedong began the Cultural Revolution shortly thereafter, an event criticized by the WPK as "left-wing opportunism" and a manifestation of the "Trotskyist theory of a permanent revolution." Relations between the CPSU and the CCP stabilized during the 1960s, with the WPK making it clear it would remain neutral in the Sino–Soviet conflict, thus resulting in the 1966 launch of the Juche program aimed at national self-determination at all levels. This, in turn, strengthened Kim Il-sung's position in the WPK. Beginning in the 1960s, Kim Il-sung's cult of personality reached new heights. It had been no greater than Stalin's or Mao's until 1972, when his birthday on 15 April became the country's main public holiday and statues of him began to be built nationwide. Kim became known as "Great Leader", the "Sun of the Nation", "The Iron All-Victorious General" and "Marshal of the All-Mighty Republic" in WPK and state publications; official propaganda stated that "burning loyalty to the leader" was one of the main characteristics of any Korean. Kim Il-sung and his guerilla faction had purged the WPK of its opposing factions during the 1950s and the 1960s, to the dismay of both the CCP and the CPSU. The domestic faction was the first to go (in 1953–55), followed by the Yan'an faction in 1957–58 and the Soviet Koreans (along with anyone else deemed unfaithful to the WPK leadership) in the 1957–62 purge. According to historian Andrei Lankov, "Kim Il-sung had become not only supreme but also the omnipotent ruler of North Korea—no longer merely 'first amongst equals, as had been the case in the late 1940s". After purging his WPK opposition, Kim Il-sung consolidated his power base with nepotism and hereditary succession in the Kim family and the guerilla faction. Beginning in the late 1980s, "a high (and increasing) proportion of North Korean high officials have been sons of high officials." Since the 1960s, Kim Il-sung had appointed family members to positions of power. By the early 1990s, a number of leading national offices were held by people in his family: Kang Song-san (Premier of the Administrative Council and member of the WPK Secretariat), Pak Song-chol (Vice President), Hwang Jang-yop and Kim Chung-rin (members of the WPK Secretariat), Kim Yong-sun