The public has been subjected to a bewildering array of opinions about modernizing the nation's intercontinental ballistic missile or ICBM force. This has lent the subject a greatly misleading aura of complexity. But neither the basic strategy of deterrence nor the approaches to provide forces to underwrite that strategy are at all complex.There is widespread agreement that the first imperative of national security is maintaining an adequate nuclear deterrent. Several administrations have agreed that the United States' strategic nuclear forces must both deter a surprise attack (the so-called bolt out of the blue) by remaining on daily alert and provide stability in the event of an evolving crisis with rapidly generated, survivable in creases in the alert force.

The ICBM controversy of the moment is whether the single-warhead, small ICBM, which some have dubbed the Midgetman, or the multiple-warhead, railroad-based Peace keeper or MX missile can best survive a surprise attack. Unfortunately, debates regarding modernization have focused far too much on a hypothetical surprise attack on the United States. Thanks to the attention and resources devoted, especially during the last decade, to addressing this possibility, we are today confident of deterring a surprise attack. Ongoing programs, properly supported, will continue that assurance in the future.

An attack against United States forces that are in a peacetime alert posture would occur - by definition - in the absence of a crisis, since a crisis produces an increased alert posture. Without a crisis, any enemy decision to initiate an attack would be the result of a deliberate calculation of gains versus risks. In the face of current and evolving United States capabilities, the risk to the attacker would be enormous.

This nation maintains its strategic nuclear forces on daily alert so that the Soviets know that any surprise attack would extract a terrible price in retaliation. But the greater challenge is maintaining deterrence in an escalating international crisis, one of such compelling pressures that the Soviet leadership might consider the consequences of retaliation less disastrous than the situation creating the crisis.

The rail garrison deployment mode for the MX - where the missile is kept in garrison on military installations during the daily peacetime alert - is carefully crafted not only to help maintain our forces on daily alert but to add stability in a crisis. It does all this at about one-third the cost of an equal number of warheads deployed on road-mobile, small ICBM's. Five hundred warheads on 50 vehicles are simply cheaper than same number of warheads on 500 vehicles.

The contention advanced by some that MX missiles in rail garrison are vulnerable because they are immobile and therefore invite attack simply ignores the facts. The Soviets cannot launch a successful attack on the MX in any situation - a surprise or in an evolving crisis. If there were ever any reason to believe that the Soviets were contemplating an attack, the logical decision would be to disperse the MX missiles, as often as required, for as long as required. In the meantime, there is simply no need to pay the daily price to maintain the system constantly on the rails.

Our progress to date in strategic forces modernization has reduced the risk of nuclear war and has also driven the Soviets' willingness to negotiate seriously at the arms control table.

(Larry D. Welch, a general, is chief of staff of the Air Force.)