YesPrimeMinister/NuclearDeterrent

"And what is the last resort?"
"If the Russians invade Western Europe."
"But you would only have twelve hours to decide. So the last resort is also the first response, is that what you're saying?"

 - Yes, Prime Minister, "The Grand Design"

: (Paraphrased from a combination of memory and the novelisation; I don't have the DVDs handy, unfortunately. - tjm)

"Postulate: Riots in West Berlin. Buildings are in flames, and the East German fire brigade are crossing the border to help. Do you press the button?"
"Well, obviously, no."
"Do you press the button if the East German police come with the fire brigade?"
[Hacker shakes his head.]
"Suppose the East Germans send some troops. Then more troops -- just for riot control, they say. And then the East German troops are replaced by Russian troops. You press the button?"
[Hacker shakes his head again.]
"Now, the Russian troops don't go. They are 'invited' to stay, to support the civilian administration. Then the civilian administration might close the roads, and Tempelhof Airport. West Berlin would be cut off. Would you now press the button?"
"I don't know! I need time to think!"
"You have twelve hours!"

 - Yes, Prime Minister, "The Grand Design"