The Fourth Republic saw continued dramatic economic growth, albeit at a slower pace than the Third Republic following the Five-Year Plans. The Park government shifted away from light industry, considered to be highly developed, and began to invest in heavy industries in a plan known as the Heavy-Chemical Industry Drive. The heavy industry shift was primarily driven by the Korean DMZ Conflict from 1966 to 1969, which caused the South Korean leadership to fear it lacked the self-sufficiency to defend itself from North Korea without significant assistance from the United States. In the late 1960s, increasing American involvement in the Vietnam War led the South Korean leadership to believe the transfer of enough troops from the United States Forces Korea to Indochina would make South Korea vulnerable to North Korea, which had amassed an enormous army, and an industrial establishment almost wholly devoted to the supply it, on the other side of the DMZ. Therefore, South Korea viewed the development of heavy industries as necessary for survival against potential North Korean aggression and set out to build an industrial infrastructure that could support a modernized military. Park decided to channel the economic development capabilities of the state into the development of several key industries: steel, petrochemicals, automobiles, machine tools, shipbuilding, and electronics. Park enlisted the "chaebols", the large family-owned conglomerates, as they possessed the capital to develop these industries. The Heavy-Chemical Industry Drive, which successfully developed heavy industry in South Korea, came at the cost of severe political and social repercussions. Wildcat strikes from the industrial working class and student protests against Park became increasingly frequent due to his undelivered promises of democratization, and are believed to have contributed to his assassination in 1979. Environmental damage and industrial accidents caused serious health issues, with one notable phenomenon known as Onsan illness, a pollution disease affecting people around Onsan, a town on the outskirts of the major city of Ulsan.