QUESTION: Why can't we use fancy ground-launched missiles to protect America from nuclear attack?

ANSWER: We promised we wouldn't. Americans and Russians have spent much of this century united in a death pact. It was one of the maddening paradoxes of the Cold War: Both sides built zillions of nuclear bombs and simultaneously agreed not to build any defense against them.The reasoning was that a good defense is dangerous to world peace. If one side ever felt invulnerable, by having constructed a defense, then it might be emboldened to start a war and rain thousands of bombs on the enemy. Thus the United States and the Soviet Union signed the 1972 Anti-Ballistic Missile treaty, each agreeing to remain totally at the mercy of the enemy. It's logical, but it's also kind of sick: We have declared that the only thing that can restrain us from obliterating millions of people on the other side of the world is the threat that we will also be destroyed.

Now, what with the Patriot missile having blasted a lot of Iraqi Scuds out of the sky, there is likely to be a newfound desire in Washington to build a nuclear defense. One of the ironies of the 1972 ABM treaty is that we're allowed to build a system that can knock conventionally armed Scuds out of the sky, but we can't defend ourselves against the vastly more destructive nuclear intercontinental ballistic missiles. ("Let this one through, boys, it's a nuke.")

The Patriot wouldn't work as a nuclear defense. A nuke isn't like a Scud. You can't destroy an incoming nuke only a mile or two from the target - it won't make much difference, you'll be fried either way. So you have to hit it farther out. What makes it all the harder is that the nuke is going to be flying much faster than a Scud, maybe 10,000 miles an hour. That bomb may also be smart, able to jerk to and fro, dodging the ABM.

John Rhinelander, legal adviser to the SALT I negotiating team, says, "The Scud is basically a 1950s missile. It comes in big, slow, dumb and happy. Patriot is a 1990s missile system. If the Patriot was up against a sophisticated offensive system, it couldn't do what it's doing."

So why not something fancier? That's the idea behind "Star Wars," the Strategic Defense Initiative. Instead of Patriot missiles, we would have lasers from space. X-ray bursts. Beam the bomb! Maybe it would work. We doubt it, though, for the same reason that all defensive systems are at a disadvantage in a nuclear world: There's no margin for error. The attacker can launch thousands of missiles and decoys at once. A single bomb can inflict intolerable damage.

There is one caveat in the ABM treaty: The U.S. and Soviets are allowed to have a single battery of defensive missiles, limited to one location. The Soviets decided to protect Moscow. We decided to protect a patch of North Dakota. No lie. We are protecting our nuclear-missile silos up there. Nuclear logic dictates that, above all, some of our bombs must be safe. Washington and New York we can do without. It's kind of moot, though: For economic reasons we stopped operating the North Dakota facility in the mid-70s.

(Why we need two Dakotas is still a mystery to those of us living elsewhere.)

Follow-up:

We want you all to know that, yes, we are aware of the "Defenestration of Prague." (Last week we wrote about the word "defenestration.")

The Defenestration of Prague - it is always capitalized - was the event that triggered the Thirty Years War that devastated much of Europe in the 17th century. (War. War. War. We can't escape it.) According to the Dictionary of Wars, by George C. Kohn, "Protestant Bohemian nobles, resisting the Hapsburg (Austrian) imposition of Catholicism, angrily tossed three Catholic envoys from Vienna, Austria, out of the window of the Hradschin Castle at Prague on May 23, 1618 . . ."

Jon Guttman, research director for Military History magazine, says, "It was traditional in Czechoslovakia (Bohemia) to break off diplomatic relations by throwing the diplomats out the window." The castle - the correct name, we believe, is Hradcany - is on a hill above the city, and the diplomats fell quite a distance, maybe 40 or 50 feet, says Guttman.

But they lived: The ground was covered in dung.

The Mailbag:

Roy J. of Rapid River, Mich., is extremely confused: "One thing I can't seem to get through my dense head is the fact that the world is supposedly round and although we may walk steadily west, we come back to our starting point from the east! And yet we have remained upright all the time. This does not make sense to me at all. If we could dig a hole here 8,000 odd miles straight down, we should come out in China or thereabouts, right? We jump into the hole feet first but we come out in China with our head uppermost, right?"

Roy, when we circumnavigate the planet we do a flip, in a sense. In fact, relative to the stars, we flip every day as the Earth turns. This is not to suggest, Roy, that you, personally, are flipped. As for the hole to China, the key thing to understand is that it is more likely to punch through someplace in the Indian Ocean. Take a snorkel.