According to AF Doctrine document 2-12, what is the second critical element required for deterrence?

The second critical element required for deterrence, as outlined in AF Doctrine document 2-12, is the WILL to use nuclear weapons. This emphasizes the notion that it is not only the possession of nuclear capabilities that deters adversaries but also the credible resolve and intent to use those capabilities if necessary. Deterrence relies heavily on the psychological aspect of influencing an adversary's decision-making process; thus, an apparent willingness to employ nuclear weapons serves as a strong deterrent against any potential aggression.

Other options such as advanced technology, global alliances, and continuous training, while important components of nuclear strategy and overall military readiness, do not directly encapsulate the essence of deterrence in the same way that the concept of WILL does. Advanced technology is crucial for ensuring effectiveness and reliability of nuclear forces, global alliances support collective security, and continuous training promotes operational preparedness, but none replace the fundamental need for a definitive willingness to act if deterrence fails.