The Senate seems inclined to delay development of an antimissile shield before it is completely effective. They should reconsider because an imperfect shield is the optimal deterrent. The United States should build a leaky umbrella and make it available to everyone.
Defense against incoming missiles should meet three criteria. Adversaries must be deterred from attacking America. All states must be deterred from launching a nuclear attack anywhere. The world must be protected against accidental launch of an armed missile.
Mutually assured destruction (MAD) deters by ensuring that adversaries face a retaliatory strike severe enough to obliterate them. How large can an initial strike be while rendering the threat of nuclear retaliation incredible? MAD is incapable of a self-enforcing nuanced response. A leaky defense provides a superior deterrent.
A completely effective missile shield disarms an adversary and guarantees that the United States does not face destruction. Such disarmament is not essential for deterrence. Adversaries must only believe that their costs in attacking the United States exceed expected gains. This is achieved by ensuring a second-strike capability sufficient to impose such costs. This retaliatory capability is guaranteed provided the shield leaks enough that missiles get through to impose unacceptable costs, which fall below complete nuclear destruction, making an all-or-nothing capability unnecessary. As nuclear weapons spread to the Third World we should note that the economic productivity and political viability of poor states could be crippled even if very few missiles reach their targets. Missile defense should guarantee such penetration, neither more nor less.
The principle of leakage to deter nuclear aggression does not apply only to rivalries. Its leakiness protects against aggression by ensuring that costs to aggressors exceed benefits. The more countries with the umbrella, the less likely anyone is to launch a nuclear missile attack.
Many wonder why it makes sense to depart from the existing policy of MAD. Besides poor retaliatory credibility, MAD offers no defense against an accidental launch. A leaky umbrella will almost certainly snare a small accidental launch.
No country should be able to strike with impunity. Defensive technology can be used to protect against the danger of an overly aggressive future leader. A leaky shield allows a small percentage of incoming missiles to hit their targets, while destroying most missiles. This establishes a credible threat of a retaliatory strike whose success is commensurate with the size of the defender. If the number of missiles getting through the shield is proportionate to the number launched, the destructive potential of the foe rises with its size. Costs and benefits are kept in balance so neither side will risk the expected punishment.
Leaks in the defense are essential to provide the stability all seek against nuclear war. Therefore, the technology should be shared and the United States should guarantee that a shield will never approach 100 percent effectiveness. Just how much leakage is optimal is a difficult question to which our best minds should turn as we negotiate the shift from MAD to a truly new world order.