Defense planners have seized on the threat of nuclear attack from nations other than the Soviet Union as a way of pumping new life into the "Star Wars" anti-missile program.
But critics of the program, formally known as the Strategic Defense Initiative, say SDI supporters are exaggerating a threat that could be dealt with in more conventional ways.
The debate comes at a time when another, unrelated anti-missile system, the Patriot, is enjoying marked success against Iraqi "Scud" missiles being fired at Israel and Saudi Arabia.
Within a decade, SDI supporters say, the following scenario of nuclear horror could occur:
* Sobered by Iraq's drubbing in 1991, regional dictators squeeze their treasuries and arm themselves with nuclear devices.
* Meanwhile, accurate long-range missiles become a staple of Third World arsenals and can strike anywhere in the world -- even the United States.
* And one day, American radar spots the flash of missiles rising through the atmosphere from a trouble spot somewhere on the globe, and the target is our East Coast.
To guard against such an attack, the Bush administration is proposing a $41 billion anti-missile plan that it says would block a limited or accidental nuclear attack.
The plan is a major change in the 9-year-old SDI program, begun in the Reagan administration to shield against a massive nuclear attack by the Soviets.
With the Soviet Union no longer so threatening, President Bush has decided to retool the program, giving it a more limited objective.
In his State of the Union message last month, Bush said SDI would be "refocused on providing protection from limited ballistic missile strikes, whatever their source."
Pentagon officials envision a combination of ground- and space-based interceptors and sensors that could foil an attack by 100 to 200 nuclear warheads.
That would be enough to guard against an accidental or renegade attack from the Soviet Union, says Edward T. Gerry, the top "Star Wars" design official.
But there is another, more ominous concern cited by program supporters.
More countries are acquiring ballistic missiles, and the range and accuracy of those missiles is likely to increase, says Gerry. Some of the countries are believed to be developing a nuclear weapons capability, as was Iraq.
As a result, he says, the possibility of a ballistic missile attack on the United States "is increasingly becoming a multilateral threat."
A Patriot-type system, while effective against relatively shorter-range, slow-flying missiles like the Iraqi Scud, would not work well against an intercontinental ballistic missile attack, says Gerry.
For one thing, he says, the ground-based Patriot was designed primarily as a battlefield weapon and defends a small area. Its "footprint" -- the area that it can defend -- is only a few miles in diameter.
It simply would be too expensive to build enough of those systems to defend the entire country, says Gerry.
In addition, the Patriot destroys an incoming missile relatively close to the ground -- a point illustrated by the damage done by falling debris in Israel and Saudi Arabia.
There are other crucial differences between a nuclear-tipped ICBM and a relatively unsophisticated missile like the Scud.
For one thing, the Scud is larger and easier to spot than a nuclear warhead. One SDI critic likens the Scud to several phone booths placed end to end, while an ICBM warhead could be compared in size to a trash can.
And the warhead from an intercontinental ballistic missile comes down far faster than a Scud, re-entering the atmosphere at some 20 times the speed of sound, according to one estimate, compared with the Scud's re-entry at six to eight times the speed of sound.
The administration's limited protection system would include:
* About 1,000 space-based interceptors known as "Brilliant Pebbles," each able to destroy a missile shortly after launch or in space, smashing into it before it had deployed its nuclear warheads.
* Ground-based missiles capable of hitting a target high in the atmosphere or above, and able to protect an area with a radius of an estimated 625 to 940 miles. Those missiles would destroy incoming warheads before they could explode.
* Sophisticated sensors on the ground and in space that would detect and track ballistic missiles and back up the anti-missile systems.
Gerry says the space-based "Brilliant Pebbles" constellation would provide global protection against a limited launch. Six ground-based anti-missile sites would protect the continental United States, along with Alaska and Hawaii.
Other types of ground-based missiles would knock out warheads headed down into the atmosphere, along with submarine launched and shorter-range missiles.
Is the technology capable of dealing with the nuclear threat, given the catastrophic results of failure?
Supporters cite the recent successful test of a ground-based interceptor built by Lockheed Missiles & Space Inc., which destroyed a target in space 100 miles above the Pacific Ocean.