Just 30 years ago the U.S. strategic deterrent was a fleet of medium-range B47 bombers, which, in a crisis, would have been moved forward to bases in Europe and Morocco and other places. Few people knew and fewer cared that all U.S. nuclear weapons were stored at two bases (in Spokane, Wash., and Limestone, Maine) and that a few enemy bombs could have destroyed the U.S. retaliatory capability. There was no sense of vulnerability.

In the 1950s the United States deployed strategic-range bombers (B52s) and medium- range ballistic missiles (Jupiter, Thor) and work was begun on submarine-launched ballistic missiles. Soon the United States had its first ICBM (intercontinental), the Atlas, and had a "triad," a three-legged deterrent involving land-based, sea-based and airborne strategic weapons.

Sputnik (1957) signaled a new era. The Soviet capacity to lift large payloads posed the question we have lived with since then: How can we measure and counter the threat against the capacity of our forces to survive a first strike?

For years U.S. bombers were based behind radar lines that could give ample warning of attack. And U.S. ICBMs were not vulnerable: The relatively poor accuracy of Soviet missiles --they could not count on landing closer than within a few miles of U.S. silos--precluded an effective attack on the silos.

For nearly three decades the United States thought in terms of deploying new technologies and then being secure for the foreseeable future. But today, with U.S. and Soviet technologies close to even, the crucial factor is the level of investment in strategic force deployments. Before SALT I, the Soviet Union was spending twice as much as the United States was. Since that misnamed arms "limitation" agreement, the Soviet Union has been spending three times as much.

The president's strategic arms procurement package comes to terms with the fact that we can no longer hope to follow the pattern of a single solution to a single vulnerability.

B52s cannot be counted on to penetrate Soviet air space all the way to targets, so B1s will be built. They will not be invulnerable-- some might be destroyed on the ground, others would not get to their targets--but Soviet planners must count on more planes getting through.

The incurable partial vulnerability of the land-based and airborne deterrents requires strengthening of the sea-based deterrent. So the Trident submarine missile, which is not large or accurate enough for a "hard-target" capability, will be replaced by one that is. By 1989, the Trident II missile will take to sea a capability comparable to today's Minuteman, the backbone of the land-based deterrent.

Adding cruise missiles to submarine and surface vessels, and using B52s and B1s as cruise missile platforms that can stand off and fire into the Soviet Union, will further challenge Soviet planners as they consider what might come back at them after an attempted "disarming" first strike on America. Cruise missiles are subsonic but small. Their low radar "cross-section" means that Soviet air defenses cannot find them by scanning vast air spaces. Cruise missiles would be close to their targets before they would be picked up, and would pose a huge air defense headache to the Soviets.

The two-part MX decision has been about two-thirds made. The missile will be produced. Fewer than half will be deployed, quickly, in extra-hardened silos. The rest will be deployed in one or more of three modes: airborne in long- flight planes; in mountain or other super-hard shelters; or in silos or shelters ringed by anti- ballistic missile defenses.

Had the president decided to concentrate on invulnerability-through-deception, using multiple shelters (4,600 shelters for 200 MXs in the Carter plan), the United States would have been committed to an open-ended expansion of the system as the Soviets added warheads to overwhelm it. The effectiveness of any strategic system can be degraded by enemy countermeasures, but some systems are more vulnerable than others to quick and severe degradation. MX in the "deceptive" mode might have been degraded quickly. The fear that the Soviets (by sophisticated satellite detection, or sensors placed in the ground by Soviet agents, or other means) might be able to locate the occupied shelters would produce a nervous, endless pursuit of possibilities of detection.

The president's package does what current technology will permit to prevent the Soviets from making any leg of the triad permanently vulnerable, and to prevent any combination of vulnerabilities from comprising an unacceptable overall vulnerability. It is the beginning of what can become the most serious strategic program in a generation.