A new missile for a new strategy?

Thanks to Congress and President Reagan, the United States has a new window of opportunity on nuclear strategy. It is an opportunity to reconsider this strategy - including such a basic concept as the triad of land, sea, and air weapons - in light of the debate spurred by MX.

Congress wisely created an MX pause by rejecting Mr. Reagan's preferred basing plan for the big new missile. Mr. Reagan wisely responded by forming a commission on strategic forces to come up with alternatives. All sides have been using the time before the commission's March report to look into basic issues of enhancing US security and deterring Soviet attack.

Now the concept of the triad is reportedly being questioned inside the Defense Department as it has long been questioned by some arms specialists outside. One argument is that the US should concentrate on a ''diad'' of submarine and airborne weaponry in which it is superior to the Soviets. This emphasis is seen as preferable to the MX or other big additions to the US land-based ICBM force. ICBMs constitute about a fourth of America's strategic arsenal but about three-quarters of Russia's. Some experts see an ironic potential loss of security for the US in pumping up a rivalry in ICBMs, the one type of arms in which Moscow does have some advantages.

By this reasoning, the US should be decelerating the ICBM race. A previous opportunity at such deceleration was lost when arms controllers failed to stop the MIRVing of missiles; that is, supplying them with multiple independently targeted warheads. The US went ahead with MIRVing, the Soviets followed suit. The MX would have 10 warheads.

Suddenly from various quarters a radical alternative is being offered. It would try to recapture the relative stability of preMIRV days. Instead of placing 10 warheads on a huge MX, it would put a single warhead on a slim ''Midgetman'' missile of not much more than a tenth the size. If both sides went back to single warheads, the vulnerability of each side would be reduced. With more and more multiple warheads, on the other hand, more targets become vulnerable to a single missile.

Even with MIRVs there are enormous uncertainties about just how vulnerable the US's ICBMs would be to an assault from the Soviet Union. Moscow could hardly be sure enough of the results to attack even the present array. Or so say those Americans who would not beef up the land leg of the triad.

Would Moscow ever consent to arms control returning to single-warhead missiles, along with the necessary provisions for verification? Certainly a phaseout of MIRVs, with the resulting lower nuclear thresholds, ought to benefit the Soviet Union, too. Such a step ought to be worth the effort of negotiators.

At the same time it may not be too late to get a handle on the new generation of weapons that could be no less destabilizing than MIRV in the future. These are the small cruise missiles that go slow compared to ICBMs but can be launched from land, sea, or air. The sea and air cruises have already turned the triad into a pentad in the lexicon of some arms specialists.

What makes them effective weapons is also what makes them dangerously destabilizing. They could be concealed on fishing boats. It would be difficult to detect whether they carried conventional or nuclear warheads. The US has reportedly decided to go ahead with the so-called ''stealth'' cruise missile, which would be fabricated for invisibility from radar above it. Even non-stealth missiles can evade ground radar and air defenses to penetrate Soviet borders.

Britain already is in the midst of controversy over deploying US cruise missiles on its soil. It is seeking at least some means to share control of them with the US.

Such pressures, along with the whole international peace movement, underscore the need for Washington to take advantage of the window of opportunity for strategic reevaluation. It must make sure that the goals of security are sought by means that do not themselves undermine it.