The controversy over the MX missile has obscured a much more serious issue--the projected expansion of U.S. capabilities for launching a disarming first strike against the strategic missile forces of the Soviet Union.

If these plans are unchecked by effective arms- control agreements--agreements much different from those proposed by the Reagan administration --we face the risk that both superpowers will put their nuclear forces on a hair-trigger, leaving a margin for error so narrow that a computer malfunction or a misread radar blip could lead to annihilation.

According to unclassified data analyzed by the Congressional Budget Office, the United States today has an arsenal of about 2,500 hard-target warheads, too few to permit a high-confidence attack of two nuclear weapons each on Soviet silos and command bunkers. By 1996, however, the combined effect of planned U.S. strategic modernization programs--the MX, Trident II and cruise missiles, along with the B1B and Stealth bombers --will triple our capabilities. The planned 1,000 MX missile warheads would account for only slightly more than 10 percent of this potent force.

Our concern about the theoretical vulnerability of our Minuteman missiles has been mitigated by our confidence in the survivability of our Poseidon and Trident nuclear submarines and strategic bombers on alert. Even after a surprise attack, we would have several thousand surviving warheads and the means for delivering them in a devastating counterattack against the Soviet Union.

From a Soviet planner's perspective, however, a U.S. capability for destroying the U.S.S.R.'s land-based missiles (over 70 percent of Soviet strategic forces), as well as submarines in port and bombers on the ground, would constitute an overwhelming first-strike threat.

Of course, the United States does not--and should not--intend to initiate a strategic nuclear exchange. But military planners everywhere are obliged to focus on capabilities rather than intentions. Just as we structure our defense on the hard evidence of Soviet capabilities rather than on ambiguous and changeable assessments of Soviet intentions, we most expect the Kremlin to do the same. The easiest, most likely Soviet response to the expansion of American hard-target forces is to put their own missiles on a quick-launch hair-trigger, the same posture our own military experts considered but rejected as too dangerous and destabilizing.

In time, the Soviets might deploy more of their missiles at sea, although they understandably have less confidence in the survivability and retaliatory effectiveness of their submarines. If instead they develop a large mobile ICBM force, the chances of obtaining tight and verifiable arms-control agreements will sharply diminish. In the meantime, with a fragile nuclear balance, we will be in a period of maximum danger.

Unfortunately, the administration's arms-control proposals will not prevent either nation from posing a first-strike threat to the other's land-based forces. Under the START plan, each side could still have 2,500 highly accurate ICBM warheads, at least three for each ICBM (up to a total of 850) allowed to its adversary. START, therefore, will not put a stop to the danger of mutual first-strike vulnerability.

The history of nuclear arms control has been disappointing precisely because arms-control agreements have consistently lagged behind the technology curve--limiting yesterday's weapons while preserving today's development programs and thus ensuring further security problems tomorrow. With the possible exception of the 1972 treaty on anti-ballistic missile systems, we have never successfully harnessed the future threats that inevitably become the present danger.

We now recognize, with the usual acuity of hindsight, the catastrophic consequences of our failure to prevent the development of multiple-warhead (MIRV) missiles in the late 1960s. Even hard-line opponents of earlier arms-control agreements now suggest a return to single-warhead missiles.

The SALT II treaty, with its limits on warheads and new missile types, would have at least postponed the period of mutual vulnerability. Now, given the rapidly developing new systems on both sides, we must devise new solutions or risk nuclear instability toward the end of this decade.

We don't have much time. We must invest the few years we do have in negotiations toward agreements conferring no first-strike advantage to either side.

It is insufficient to limit our focus to symbolic objectives like numerical equality, lower force levels and matching capabilities. Instead, we should concentrate on achieving crisis stability where neither side could gain advantage by striking first and where both have confidence in their retaliatory forces. To achieve such stability, we have to link our weapons programs directly to our negotiating strategy so that neither contradicts or cancels out the other.

Until and unless we make this linkage, our strategic programs--and, similarly, new Soviet systems-- will only exacerbate, rather than solve, our security problems. Defense programs unrestrained by consideration of likely Soviet responses and their consequences will, ironically, weaken deterrence. And the tens of billions of dollars spent on these new technologies will result ultimately, not in American superiority, but in greater mutual insecurity.