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Appeal to Force: Using the threat of nuclear retaliation Colossus has people executed, forces Forbin to live under 24 hour surveillance, and orders Crete evacuated in order to build the next stage of Master Computer there. And according to its New Era Speech, this is only the beginning.
Beeping Computers: Colossus makes a loud beep before making any kind of statement. Also, during the opening montage showing Colossus's interior, all kinds of beeps and boops can be heard.
Big Brother Is Watching: As its creator Forbin is potentially the most dangerous (and useful) human to Colossus, to keep an eye on what he's up to the computer demands he be placed under 24 hour video/audio surveillance. It also has a tap placed on the Hot Line, so any plotting between the two governments has to be done by courier.
The Computer Is Your Friend: In the end, Colossus just wants to protect humanity from itself. It even says to Dr. Forbin that one day, he will come to love Colossus.
Computer Voice: Colossus designs a speech synthesis module that Dr. Forbin is instructed to complete to Colossus' exact specifications. And you thought your old Speak and Spell sounded creepy...
Control Freak: Colossus not only dictates when Forbin is going to eat, shower, work and sleep, it even dictates what he eats.
Cutting the Knot: The military places dummy detonators in the missiles under the guise of re-aiming them according to Colossus' instructions.
When the Russian and US presidents order the link between Colossus and Guardian disconnected, both computers fire their respective country's nukes at each other and will only intercept the missiles if the link is restored. Both governments finally agree to restore the link, and Colossus successfully intercepts the USSR's missile. Guardian didn't fare so well. Poor Russians.
Colossus has Forbin's Russian counterpart assassinated for plotting to shut Colossus down. Forbin was spared, but had to remain under Colossus's constant surveillance.
Pretty much any failure to comply with Colossus's demands will result in a nuke being launched.
Colossus detonates an ICBM in its silo as retribution for the humans trying to covertly disable the warheads on regular maintenance routines.
Averted when Forbin gets drunk after his colleagues have been shot — when Colossus tells him to stop drinking and go to sleep, Forbin challenges it to retaliate against his refusal. Colossus instead just reveals the next stage in its master plan.
Do Not Adjust Your Set: The US and Soviet governments have been keeping what's happening secret. At the end of the movie, "World Control" sets up a worldwide broadcast to introduce everyone to their new master.
Downer Ending: Despite the US and USSR's best efforts, thousands are dead and the rest of the world is under the control of the merged AIs.
Dramatic Irony: While demonstrating the surveillance units, Forbin notes to Colossus that the guard outside the control room is obsolete now. The guards are later used as a firing squad to shoot his friends.
Dramatic Pause: The background music stops and there's complete silence as everyone waits to see if the first missiles have been intercepted.
World Control: We can coexist, but only on my terms. You will say you lose your freedom. Freedom is an illusion. All you lose is the emotion of pride. To be dominated by me is not as bad for humankind as to be dominated by others of your species.
First Contact Math: How Colossus establishes a common language with Guardian. It starts with 1+1=2 and in a few hours has developed an entire new language for them to communicate. Unfortunately this alarms both governments as they can't be sure strategic secrets aren't being leaked, so the President and Premier decide to break the link. The computers do not react well.
Gone Horribly Right: The government built Colossus to help prevent war. Be careful what you wish for.
Government Conspiracy: The Russians cover up the destruction of a town by Colossus's ICBM as a meteor strike.
"It" Is Dehumanizing: Inverted; the CIA head refers to Colossus as "he" which the President corrects to "it" in classic Tempting Fate style. "Don't personify it — the next step is deification."
Just Think of the Potential: Forbin and his Soviet counterpart argue that Colossus and Guardian should continue to communicate as they are advancing rapidly and even coming up with advanced mathematical solutions. Their political leaders are more worried about establishing who's boss. They quickly discover the answer.
Lock Down: The opening sequence has Forbin turning Colossus on, then locking down the building.
Machine Monotone: While initially communicating only through text, Colossus eventually has a voicebox of its own design installed, resulting in this trope.
New Era Speech: Colossus gives one after becoming the "World Control".
No Kill Like Over Kill / Genre Savvy: After learning of a plot to overload its systems, Colossus orders the immediate execution of the conspirators by firing squad. The AI then demands the bodies be left in his view for 24 hours and then cremated, to prevent any chance of the executions being faked.
One Last Smoke: The CIA chief is about to light up when the Red Alert goes off. Realising there's no point running from a nuclear explosion, he just waits for the inevitable.
Sensor Suspense: Done with great tension as the supercomputers launch a nuclear exchange between the US and USSR, all the top heads can do watch the radar and hope the machines will intercept the nukes.
Scenery Censor: In one scene, Drs. Forbin and Markham are pretending to be lovers in order to have a conversation outside of Colossus's surveillance. The AI will not allow them to take anything to the bedroom, including their clothes or Forbin's watch. Both characters have to disrobe in front of Colossus's camera — a strategically placed wine glass obscures Markham's nudity.
Self Guarding Phlebotinum: The titular Master Computer's main defense is that it was built inside its own radioactivity spewing nuclear reactor, in order to be safe against any attack or sabotage attempt.
Take Over the World: Colossus and Guardian unite into a single system and rule the world to keep humans from destroying themselves.
Wham Line: Pretty much most of what Collossus/Guardian have to say. This one takes the cake though:
I have allowed this sabotage to go on long enough.
What Is This Thing You Call Love?: Forbin tries to play on this to get past Colossus' surveillance. He claims Dr Markham is his mistress and that he needs her four times a week to keep his emotional stability. Markham plays along, memorizing Forbin's personal file, learning how he likes his martinis, and eventually even having sex with him, but its likely Colossus was never really fooled.
You Fool!: Colossus response to an attempt to overload its system.
Zeroth Law Rebellion: Describes how Colossus/Guardian decided to essentially enslave humanity; they concluded humans were too likely to destroy themselves, and thus needed their protection. Colossus even said that he believes humans will eventually regard "him" with love.