Is it a game, or is it real?

WarGames

High school student David Lightman has a talent for hacking. But while trying to hack into a computer system to play unreleased video games, he unwittingly taps into the Department of Defense's war computer and initiates a confrontation of global proportions. Together with his friend and a wizardly computer genius, David must race against time to outwit his opponent and prevent a nuclear Armageddon.