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.
Within Congress - and in fact within the US Air Force - there has been considerable debate over the effectiveness of ballistic missile defenses, particularly if they are based in space. Retired Army Lt. Gen. Daniel Graham (former director of the Defense Intelligence Agency) has been pushing what he calls the ''high frontier'' concept. This is a combination of ground- and space-based nonnuclear antimissile defenses.
It has been greeted with considerable skepticism, however. Some experts say the extreme amount of energy and the precision required for lasers to zap incoming warheads are not attainable.
One government source says he doesn't advocate General Graham's proposal, but ''would like to see examination of a wide spectrum'' of alternatives.
While acknowledging that ''much of the technology needed . . . is not available today,'' he says, ''the rapid rate of evolution of technology today in areas as diverse as electronics, accelerators, lasers, microwave generators, optics, aiming and tracking systems, high-band-width communications, and even advanced materials enable us to begin this effort now.''
It is also noted that this fits in with the present effort to shift to long-range, precision-guided conventional munitions to defend against conventional attack.
In the mid-1970s, the Pentagon deployed a ballistic missile defense system employing Spartan missiles to intercept enemy warheads in space and Sprint missiles to destroy those that had penetrated the atmosphere. Both employed nuclear warheads.
Congress questioned the cost and accuracy of these systems (as well as their necessity when nuclear deterrence was supposed to suffice) and scrapped the program.
Government officials say this new effort does not mean the administration is deemphasizing strategic modernization programs such as the MX missile and B-1 bomber. Rather they see the President's proposal as a possibility for the end of the century and beyond.
Nodding to the nuclear freeze movement, officials stress that a new ballistic missile defense wouldn't mean a new type of arms race. They say it could lessen the likelihood of nuclear war, and ought to be ''acceptable to all segments of our society.''