WarGames (1983)

In director John Badham's sci-fi fantasy:

• the scene of young computer-game player/hacker David Lightman (Matthew Broderick) challenging NORAD's military super-computer (WOPR - War Operations Plan Response) to a 'simulated' game of Global Thermonuclear War ("SHALL WE PLAY A GAME?...Love to. How about Global Thermonuclear War?") - not realizing that the computer was connected to the US arsenal of ICBM missiles
• the tense scene in the NORAD headquarters where David helped to 'teach' the computer to 'learn' how to play itself in an endless series of tied Tic-Tac-Toe games - thereby getting it to stop its play (by understanding the concept of mutual assured destruction with no winners)