Brief (not very good) synopsis:
Thinking this will prevent war, the US government gives an impenetrable supercomputer total control over launching nuclear missiles. But what the computer does with the power is unimaginable to its creators.
Longer, a bit spoilery synopsis:
spoiler
Forbin is the designer of an incredibly sophisticated computer that will run all of America's nuclear defenses. Shortly after being turned on, it detects the existence of Guardian, the Soviet counterpart, previously unknown to US Planners. Both computers insist that they be linked, and after taking safeguards to preserve confidential material, each side agrees to allow it. As soon as the link is established the two become a new Super computer and threaten the world with the immediate launch of nuclear weapons if they are detached. Colossus begins to give its plans for the management of the world under its guidance. Forbin and the other scientists form a technological resistance to Colossus which must operate underground.
Somewhat cheesy trailer: https://www.youtube.com/watch?v=SmSsXoPxi0M
One of my all time favorite science fiction films, and a surprisingly prescient one. It's a film that assumes its viewers are intelligent people and doesn't feel like it needs to spoon-feed them everything. It's very much about letting the story unfold through realistic dialogue.
It's also beautifully shot in the super wide 2.35 : 1 Cinemascope format.
The cast was mostly unknown at the time, although National Treasure James Hong (always use his full title) has a small role, as does Marion Ross, later to be the mom on Happy Days. The lead, German actor Eric Braeden, went on to have an extremely long career on the soap opera The Young and the Restless.
The director is Joseph Sargent, who I always think of as the director of the Star Trek TOS episode The Corbomite Maneuver, but he's probably more known as the director of The Taking of Pelham One Two Three. It's based on the first of three novels by D. F. Jones and the screenplay, written by James Bridges (The Paper Chase, The China Syndrome) is surprisingly faithful to it.
Also, a cool bit of trivia: Every piece of computer equipment in the film was functional and they had to have a huge AC to cool it all.
It is watchable on the Internet Archive. I'm guessing linking to it is not okay, but you can find it there.
Runtime: 1 hour 40 minutes.
And now, I'm off to rewatch it!