Why did NATO adopt flexible response instead of relying only on an all-out nuclear response?

NATO adopted the doctrine of flexible response in 1967 to move away from an "all-or-nothing" nuclear strategy, which had become increasingly difficult to justify as a credible deterrent.12

During the early Cold War, the United States and its allies relied on the threat of massive nuclear retaliation to deter Soviet aggression. However, as the Soviet Union developed its own nuclear capabilities, the concept of mutual assured destruction (MAD) emerged. Because a full-scale nuclear exchange would be suicidal for both sides, U.S. leaders doubted that any president would realistically choose to initiate a nuclear strike in response to conventional provocations.123

Flexible response addressed this credibility gap by:
* Staged Escalation: Envisioning a response that began with conventional forces to contain an initial invasion, thereby preventing the immediate transition from the first shots of a conflict to a nuclear holocaust.12
* Linking Conventional and Nuclear Forces: Maintaining a clear connection between a land war in Europe and the U.S. strategic nuclear arsenal, ensuring that the threat of nuclear escalation remained a deterrent against conventional war.12
* Enhancing Credibility: Using battlefield nuclear weapons to signal resolve, creating uncertainty for the Soviet Union regarding the threshold at which NATO might resort to nuclear use.2