THERE are many theoretical ways to overcome a strategic defense system, but increasing the size of the attacking missile force is at the heart of every one of them. Viewed another way, agreeing to cut offensive missile warheads in the face of a negotiating partner's commitment to deploy a strategic defense is not merely a bargaining concession, but rather an act of national treason. A strategic force potentially halved by a START accord, further thinned out by a first strike, and then mopped up by a deployed Strategic Defense Initiative (SDI) system is one that has lost much of its value as a deterrent. Its proprietor has been consigned to second-class status in the nuclear superpower club. That is why the Soviets cannot live with both SDI and START.
So let us not labor the obvious, that which is known to every strategic thinker in the United States, every US START negotiator, and everyone substantively involved in the Reagan-Gorbachev summit: The odds of the Soviet Union's making deep cuts in offensive nuclear missiles in the face of imminent SDI deployment by the US is zero.
What then do we make of the ``agreement to disagree'' over the pertinence of the 1972 Antiballistic Missile (ABM) Treaty to SDI testing during the non-withdrawal period to be negotiated? Would the Soviets actually sign and enter into a START accord with the SDI issue unresolved? And would such an accord be ratified by the Senate?
Those who have been in periodic touch with Soviet strategic planners during the past four years say their thinking about dealing with SDI has evolved materially during that period.
When the President first declared his initiative in 1983 Moscow's reaction was one of anxiety. At least one key planner, Marshal Sergei Akhromeyev - now chief of staff - privately told visitors he thought the US had already achieved a major breakthrough in the SDI area or else President Reagan would not have put himself so far out on the limb.
The result of this anxiety was the frontal Soviet assault on ``star wars,'' culminating with General Secretary Mikhail Gorbachev's effort at the Reykjavik summit to swap cancellation of the system for deep offensive missile cuts.
A number of factors have now combined to change both Soviet thinking and tactics. Clearly their Reykjavik approach was a political nonstarter, generating scant support either here or in Europe. Moreover, a number of reputable studies have shown SDI to be more of a product of right-wing ideology and military hype than scientific progress.
At the same time, US budgetary cutbacks and congressional resistance to an administration interpretation of the ABM Treaty at variance with the agreement's plain language barring the testing of ABM components in space further influenced Soviet tactics. No longer was a direct assault on the treaty needed. Merely a commitment to observe the ABM accord could suffice.
One approach, supported by some within the administration, involved agreement in advance on the kinds of experiments permitted under the ABM Treaty - the so-called ``threshold'' method based on such criteria as the intensity of experimental lasers, the size of space mirrors, and the speed of missile interceptors. US observance of such testing thresholds could provide the Soviets with insurance against a US ``breakout'' at the conclusion of the ABM Treaty observance period.
As a fallback position, the Soviets might sign a START agreement that sanctified termination by one party in the face of a material breach by the other. This would be coupled with a unilateral Soviet declaration that the testing of SDI components in space would constitute that material breach.
It bears emphasis that while the Soviets might conceivably sign a START accord with the ABM/SDI issue unresolved, they would without question withdraw from a commitment to deep offensive missile cuts in the face of a clear US intention to deploy SDI.
Thus the proposal of the secretary of state and other SDI advocates to abandon the fight over ABM interpretation and approach SDI tests on a case-by-case basis is rejected by many on Capitol Hill as a formula for instability - a strategic roll of the dice. Why ratify a treaty whose central issue is unresolved? How does one make sensible decisions about force structure on the basis of assumptions that may be torn asunder within half a decade? How can matters of such consequence be made hostage to Moscow's interpretation of whether one weapons test or another violates a decades-old treaty?
The issue is not lasers, sensors, boosters, computers, or other SDI hardware or software. These technologies must and will be developed both as a hedge against Soviet breakthroughs and because they are important to other weapons programs. But the bottom line for Ronald Reagan appears now to be that he cannot have both deep nuclear missile cuts designed to further strategic stability and an SDI testing program that throws such stability into a cocked hat.
C. Robert Zelnick is Pentagon correspondent for ABC News and former Supreme Court correspondent for the Monitor and National Public Radio.