The First Seven-Year Plan (initially 1961–1967) built on the groundwork of the earlier plans but changed the focus of industrialization. Heavy industry, with the machine tool industry as its linchpin, was given continuing priority. During the plan, however, the economy experienced widespread slowdowns and reverses for the first time, in sharp contrast to the rapid and uninterrupted growth during previous plans. Disappointing performance forced the planners to extend the plan three more years, until 1970. During the last part of the "de facto" ten-year plan, emphasis shifted to pursuing parallel development of the economy and of defense capabilities. This shift was prompted by concern over the military takeover in South Korea by General Park Chung-hee (1961–1979), escalation of the United States involvement in Vietnam, and the widening Sino-Soviet split. It was thought that stimulating a technological revolution in the munitions industry was one means to achieve these parallel goals. In the end, the necessity to divert resources to defense became the official explanation for the plan's failure.