WarGames (1983)

High school student David Lightman (Matthew Broderick) unwittingly hacks into a military supercomputer while searching for new video games. After starting a game of Global Thermonuclear War, Lightman leads the supercomputer to activate the nation's nuclear arsenal in response to his simulated threat as the Soviet Union. Once the clueless hacker comes to his senses, Lightman, with help from his girlfriend (Ally Sheedy), must find a way to alert the authorities to stop the onset of World War III.