SOVIET preparations at the Reykjavik summit focused on carrots to tease limitations on the Strategic Defense Initiative (SDI) out of President Ronald Reagan, while American tactics undoubtedly included layers of protection to limit concessions and to prevent damage to the program. But the two leaders were looking exclusively at the two ``standard'' versions of SDI: ``SDI I,'' the total shield that President Reagan proposed, and ``SDI II,'' which has the much closer goal of a quick deployment to protect United States missiles and bust the Anti-Ballistic Missile Treaty once and for all. Neither SDI I nor SDI II is likely to be effective soon, except at destroying the ABM Treaty. But two new parallel programs can be envisioned -- SDI III and SDI IV -- which might hold out real promise for increasing the security of both countries in less than five years.
Construction of SDI I is an option only in the next century, if ever. Even if it were built in the near future, SDI II would serve the American people poorly. Soviet planners might perceive that a near-term missile defense, being leaky, could be overwhelmed by modest increases in offensive striking power. The Soviets might then choose to offset a US defense by increasing the number of warheads aimed at US missiles.
If the Soviets overestimate the performance of our defense, while prudently underestimating the capabilities of their own missiles, more warheads might hit US targets than would if this country remained defenseless. Furthermore, the Soviet Union will never permit a unilateral American action to disarm its strategic forces.
Fortunately, a modest investment in the SDI III and SDI IV programs would afford great protection to this country at low cost and in short order. SDI III could be deployed in only five years with no extraordinary research effort; it would decrease by at least half the dangers facing us. SDI IV would take longer, but would eliminate our fears, and perhaps those of the Soviets, of a disarming first strike.
SDI III is known as strategic arms control. Both the US and the USSR have expressed a willingness to cut their offensive forces in half, the proposed effectiveness of SDI III. Their offer to us is not acceptable, but neither is ours acceptable to them. In betweeen there may be common ground. This is even more likely since some agreement on 30 percent reductions may be possible.
Achieving such deep reductions probably requires renunciation by both sides of any efforts to achieve SDI II, which is a good bargain. Any nuclear warhead we can negotiate out of the Soviet stockpile is a weapon that has been engaged and destroyed with a kill probability of 100 percent; it is unlikely that any defense will ever achieve such success.
Fifty percent force reductions appear at first glance to be militarily equivalent to keeping today's offensive forces while building a defense that is 50 percent effective. In fact, reductions are better. Few weapons achieve a 50 percent success rate on the test range. Very few of those weapons that are 50 percent effective on the test range turn out to be better than 10 percent effective the first time they are used in combat; there will not be a second nuclear battle for which we can fine-tune our defenses, having learned from experience.
SDI IV is simply improvement of the survivability of our retaliatory forces. In the last year or so it has become clear that many techniques exist to protect even land-based intercontinental missiles from attack. Superhardened silos work very well indeed. The ``Carry Hard'' combination of mobility and sheltering for the MX missile will probably work. At present Soviet force levels, it is impossible for them to attack a Midgetman mobile missile system, even one confined to federal lands already dedicated to missile deployments and other dangerous activities. With SDI IV in place and offensive force levels cut in half, attacks on land-based missiles would become absurd.
If Soviet multiwarhead missiles were deprived of their American targets, not by defenses that could be overcome by mass and power, but by deployments that approached true invulnerability, our potential enemy might decide to spend his rubles on less destabilizing weapons, or use his resources to improve his society. If both sides decided to retain limited ICBM forces, SDI III would improve the chances for the survival of our societies, should deterrence ever fail.
SDI III will not provide a leakproof umbrella to keep out nuclear missiles, but, combined with SDI IV, it should produce a climate in which both sides feel that they can further reduce their strategic forces. Indeed, with some experience and confidence in the new era, force levels might well fall so low that plausible defense systems could handle the threat.
Then, as President Reagan once suggested, when nuclear forces have almost been eliminated, SDI I could be reasonably developed and deployed. But that time is still far in the future. The time for SDI III and SDI IV is here; and the summit provides a good opportunity to begin their ``development and deployment.''
Peter D. Zimmerman is senior associate, Carnegie Endowment for International Peace.