Mutual deterrence dissolves when one or both sides can remove the opponent’s ability to inflict severe damage in retaliation. In a crisis the side believing that an unacceptably large part of its retaliatory capacity could be suddenly nullified by an opponents forces might be impelled to strike preemptively. The superior side also might be motivated to undertake the first aggressive actions. Regardless of its original intentions, the stronger side could plausibly imagine a siege mentality, a use-them-or-lose them attitude, operating on the weaker side, and reason, rightly or wrongly, that it had better seize the initiative. A condition of vulnerability on both sides would further strengthen incentives for preemptive attack and pose an unacceptable risk of nuclear war. From the perspective of deterrence theory this is the worst of all hypothetical worlds.
Blair, Bruce G. Strategic Command and Control: Redefining the Nuclear Threat. 1985. p. 17 (hardcover)
This is why — as Ronald Reagan never understood — attempts at ballistic missile defence are fundamentally destabilizing.
These dynamics probably also increase the risk of nuclear war between India and Pakistan.