What is the core technical rationale behind limiting the deployment of Anti-Ballistic Missile (ABM) systems?

The core technical rationale behind limiting the deployment of Anti-Ballistic Missile (ABM) systems is to preserve the credibility of nuclear deterrence by ensuring that a potential aggressor cannot eliminate the retaliatory capability of the other side, thus maintaining Mutual Assured Destruction (MAD). If a country develops a highly effective ABM system capable of intercepting incoming nuclear warheads, it might be tempted to launch a first strike, believing it could absorb any retaliatory attack. Conversely, if one side fears that the other is developing a robust ABM system, it might feel compelled to increase its offensive nuclear forces to overwhelm the ABM defenses or to launch a preemptive strike in a crisis. This action-reaction cycle could lead to an arms race and increase the risk of nuclear war. The ABM Treaty of 1972 between the United States and the Soviet Union, for example, limited each side to only two ABM sites (later reduced to one) to prevent either country from developing a nationwide ABM defense. By limiting ABM systems, both sides remained vulnerable to nuclear retaliation, which theoretically deterred either side from initiating a nuclear attack. Therefore, the rationale for limiting ABM systems is based on the strategic stability achieved through mutual vulnerability, rather than on the technical effectiveness or ineffectiveness of ABM technology itself.