President Reagan's dramatic proposal to build a new ballistic missile defense system brings to the public and political domain the growing debate among experts over how a nuclear war would likely be fought and why it may be getting increasingly difficult to prevent.
It is an admission that intercontinental missiles are becoming (paradoxically) so threatening, yet so vulnerable, that a first strike by one of the superpowers is now conceivable, at least among war planners and strategic theorists. It parallels the debate over the MX and increasing calls (most recently from Henry Kissinger) for the United States and the Soviet Union to move to smaller, mobile missiles while working for eventual deep strategic-arms reductions.
The administration sees this as its equivalent to President John F. Kennedy's call to put a man on the moon within 10 years.
''That's a very good example of how quickly America can achieve things that have been felt to be impossible when the full strength of our very considerable resources are deployed behind them,'' Defense Secretary Caspar W. Weinberger told reporters traveling with him in Spain.
The administration in fact wants to spend very large sums on exploring new ballistic missile defense (BMD) systems. It has already directed much more money than its predecessor on development of ground-based BMD systems and airborne antisatellite weapons as well as lasers, particle beam devices, and other space-based offensive and defensive systems.
It is likely to shift funds within the already proposed 1984 Pentagon budget, and Secretary Weinberger predicts ''all sorts of changes in 1985 and 1986'' in this regard.
The Soviet Union was quick to charge that the President's proposal would violate the 1972 Antiballistic Missile (ABM) Treaty, which is part of SALT I (the first Strategic Arms Limitation Treaty). But Washington retorts that the treaty addresses deployment only, not research and development, and notes that the Soviets have been pressing ahead with such systems themselves.
Sources say, however, that if new BMD systems are developed, the ABM treaty might have to be scrapped in favor of ''a more comprehensive arms-control regime.''
US officials deny that this is an effort to develop a ''fortress America'' and abandon its European allies. In fact, they say, such systems could protect allied countries from the threat of intermediate-range nuclear missiles aimed at them.