In the nearly 40-year fight over building weapons to shoot down incoming missiles, the proponents have generally fallen into two camps, the dreamers and the schemers.

When the idea of missile defense had its most celebrated moment under President Reagan, the dreamers - including the president and the renowned nuclear scientist Edward Teller - seemed convinced that we could be made invulnerable against nuclear weapons. The more cynical camp - including the national security adviser, Robert McFarlane, and the military assistant to the secretary of defense, Colin Powell - saw an impregnable defense as a pipe dream, but also a useful bargaining chip. It wouldn't stop a nuclear strike, but it would worry the Soviet military planners, and make it easier to drive a favorable deal in arms control talks.

That time around, the schemers had it right. The impermeable superdome was a technological fantasy, and one that could have bankrupted the national treasury. Even if it had worked, it would have been dangerous, because it would have encouraged the illusion that we could win a nuclear war. The prospect of an American missile defense system did, however, help goad the Soviets into mutual cuts in our nuclear arsenals.

Now, too, there are dreamers and schemers. The dreamers, possibly including the president, embrace missile defense at face value, as something that will make us safer in our beds. Such a system, they assert, will protect us against a terrorist with a ballistic missile, an accidental launch from the aging Russian arsenal, or a rogue state bent on demolishing an American city. The public debate so far has been almost entirely about this dream of missile defense, which - because it aims to stop a small flock of missiles rather than Russia's thousands - is technologically more plausible than what President Reagan had in mind.

The schemer agenda, on the other hand, is about nuclear strategy, a forbidding subject framed in arcane and speculative language that tends to scare off laymen. But let's see if we amateurs can get our heads around it.

The concept at the heart of nuclear strategy is deterrence, which means that our ability to obliterate the enemy prevents him from doing something rash. It is generally accepted that our nuclear strength deterred the Soviet Union from raining nuclear warheads on America. But preventing Armageddon was not the main purpose of our nuclear forces. The foremost purpose was to stop the Soviet Union from sending its superior non-nuclear armies into Western Europe. By deliberately leaving open the possibility that we would go nuclear if Soviet tanks crossed the Fulda Gap into West Germany, we deterred the Soviets from beginning a conventional war in Europe. Would we in fact have risked decimating the planet to save Europe? Maybe not, but the Soviets could never be sure.

The schemers in the current debate fear that any nation with a few nuclear weapons can do to us what we did to the Soviets - deter us from projecting our vastly superior conventional forces into the world. This could mean Iraq or North Korea or Iran, but it most importantly means China. The real logic of missile defense, to these advocates, is not to defend but to protect our freedom to attack.

There was a funny misfire of a debate about deterrence earlier this year. President Bush, arguing the need for missile defense, suggested that a rogue state might not be restrained by the fear of nuclear annihilation, the way the Soviet Union was. Critics pounced gleefully: wouldn't North Korea or Iraq be deterred from launching an unprovoked attack, just as the Soviet Union was, by the certain knowledge that we could reduce them to molten rubble? Well, sure they would. Unless we happened to have our tank divisions parked at the outskirts of their capital, prepared to move in. Under those circumstances, even a semi-rational megalomaniac like Saddam Hussein might just decide to launch whatever he had. Or, more to the point, we couldn't be quite sure he wouldn't. If Saddam had possessed a nuclear missile in 1991, could we have persuaded such a broad coalition to drive him from Kuwait? Or, if the Taliban had a single missile capable of pulverizing Washington, would we have been so quick to go into Afghanistan?

You won't hear President Bush saying so, but the scenario that preoccupies many of those in and around the Pentagon is this one: Taiwan decides to risk a climactic break with mainland China. The mainland responds with a military tantrum. America would like to defend the island democracy against the Communist giant - but we are backed down by hints that Beijing cares enough about this issue to launch nuclear missiles. American voters may or may not support a conventional war for Taiwanese independence; they're much less likely to support one that risks the obliteration of our cities. Ah, but if we have an insurance policy, a battery of anti-missile weapons sufficient (in theory) to neutralize China's two dozen nuclear missiles, we would feel freer to go to war over Taiwan.

"The logic of missile defense is to make the stakes of power projection compatible with the risks of power projection," says Keith B. Payne, a deterrence theory expert and an ardent supporter of missile defense. Missile defense, in other words, is not about defense. It's about offense.

This debate about missile defense is one we're not having. The schemer rationale exists mostly between the lines. It is implicit in documents no mere citizen reads, like the Quadrennial Defense Review, and encoded in speeches. There is little frank discussion of it in publications for non-specialists. (One exception is the right- wing National Review, whose editor, Richard Lowry, has articulated the force projection rationale clearly.)

Why is everyone being so coy about this?

For one thing, the dreamers' just-plain-defense argument is easier to grasp, and much easier to market. In principle it's hard to argue that a system that could shoot down a rogue missile or two would be a bad thing to have. Even liberals are buying into it. Their reservations are framed almost entirely as variations on: Is it worth the cost? Can we afford the money to make the thing work? Is it a better value than the alternatives? Is it worth the political angst of withdrawing from the ABM treaty?

Personally, if missile defense is about defense, I can imagine better ways to spend $100 billion. Defending our porous seaports against a nuclear device in a tugboat or shipping container seems like a more urgent investment. And if we're really worried about an accidental launch from a decaying Russian missile command center, we might revive a bright idea the physicist Sherman Frankel developed a decade ago - retrofitting nuclear missiles, ours and theirs, with devices so they could be disarmed and destroyed after a mistaken launch. (Incredibly, civilian rockets have post-launch destruct devices, but not nuclear missiles.) But after Sept. 11, the public is less likely to quibble over priorities and cost-benefit analysis. If it makes us feel safe, the mood is, buy it.

The schemers' agenda, on the other hand, makes a more complicated and uncomfortable debate, because it raises the question of whether missile defense might, in fact, make the world less safe. "Force projection" has an unpleasant, bellicose ring to it. It also drives the Chinese up the wall. There are already plenty of hawks in China who believe we have a long- range strategy to "contain" it - and the force projection rationale tends to suggest they are right.

Arguing that we need missile defense to assure we can take the battle to the nuclear-armed bad guys opens up two ticklish lines of discussion.

One is whether missile defense makes it likelier we will get into a war that is not essential to our national interests, or that we will move more easily from containing bad regimes to ousting them, and whether as part of such a conflict we may find ourselves playing nuclear chicken.

The other is whether missile defense might lead to a new arms-building competition. If it is true that China cares enough about Taiwan to threaten nuclear war - that is, if China's ability to deter us with nuclear weapons really matters to Chinese leaders - then it stands to reason they will work hard to protect their deterrent. However they do that, by manufacturing more missiles or putting multiple warheads on each launcher or by a shift in strategy, a Chinese buildup may well influence the behavior of China's wary nuclear neighbor India. What India does in turn alarms its nuclear neighbor Pakistan. If you're following the news, you know that India and Pakistan are at this moment on the verge of war.

Strategic planners have a technical expression for this kind of discussion. It's called a can of worms.

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