From Wikipedia - "In the Terminator universe's original timeline, an undisclosed series of events leads to Cyberdyne's development of Skynet, a network of supercomputers that employ artificial intelligence in order to replace human beings as commercial and military aircraft pilots, and for the control of other military systems, including nuclear missiles. On August 29, 1997 ("Judgment Day"), Skynet becomes sentient, humans attempt to shut it down, and Skynet retaliates by launching a nuclear attack, initiating an indeterminately long period of global warfare that pits humans against machines with ever-increasing capabilities."