I didn't understand what Colossus-Guardian was trying to do by destroying those nukes in the US and USSR at the end. Is that related to the humans replacing the nukes with bogus parts? How did the computers foil that by destroying 2 missile sites?
The Computer said that it had known all this time that the humans were trying to dismantle the nukes. The Computer chose to destroy the nukes that were then currently being dismantled - as a warning. This was a warning that:
a: The computer knew where and when the humans would try to dismantle the nukes. b: kill local humans as a punishment for trying to dismantle the nukes. c. warning that the nukes will be detonated on home soil if the nukes will ever be tampered with again.