Originally Posted by Blademaster

That's because they realized that the US would use nuclear weapons against Warsaw Pact. They thought, "For god' sake, they dropped two nuclear bombs on an already defeated Japan, simply just to save lives, not out of desperate survival. What makes it so sure that they wouldn't do the same thing to us?"

USSR literally took Nixon's threat seriously. When they saw the US's committment during the Berlin Airlift, all bets were off. In poker parlance, they realized that the hands they were holding were not worth risking the pot that they had based on US and NATO's posture.