In her book "The Guns of August," Barbara Tuchman says that before World War I, the French believed in something called ,elan -- a spirit, a dash, a boldness that was supposed to be uniquely theirs. Elan is what led the army to contemptuously reject khaki and retain red pants. "Eliminate the red trousers?" cried a former war minister. "Never!" And so soldiers died because they looked like bull's-eyes.

Most nations have their delusions, and America is no exception. Ours is called "can do" -- the belief that enough money and energy can accomplish anything. The pantheon of "can-do" triumphs include the building of the atomic bomb, the invasion of Normandy and, even, victory in World War II itself. Give GI Joe and Rosie the Riveter the goods and they'll deliver.

Now we are embarking on the grandest of all "can-do" projects -- the Strategic Defense Initiative, better known as "Star Wars." President Reagan says it promises much -- nothing less than eventual elimination of nuclear weapons. The president believes this so fervently that he is willing to bet trillions of dollars on it and, among other things, your life.

There is, after all, only one thing "Star Wars" will destroy for certain and that is the nervous understanding known as deterrence. For a doctrine based on retaliation, we would substitute one based on defense.

You can understand why the president is enamored of "Star Wars." It promises that we will make atomic war no more, that we can throw a net into space so that no missiles will get through and that, as a result, there will be no need for missiles at all. "Star Wars" will do to atomic weapons what shining armor did to the bow and arrow. Hold a room in the museum for the MX.

"Star Wars" is not the conceptual breakthrough that the president thinks it is. Instead, it is a misapplication of the old can-do ethic. The president is of the World War II generation, and it's not surprising that maybe his most moving speech was delivered on the beach at Normandy. Here is a man shaped by his times, nurtured on Edison and Bell and, probably, that nearly mythical moment when Franklin Roosevelt opened a letter from Albert Einstein and concluded in the manner of Andy Hardy, "I know, let's build an atomic bomb."

But "Star Wars" is neither an attempt to build a bomb nor a massive invasion of France. Neither of those endeavors required absolute perfection. The Normandy invasion was so vast it smothered its mistakes. As for the A-bomb, nothing much would have happened if, in fact, nothing much had happened. We could have tried again.

But that's not the case with "Star Wars." It shreds the doctrine of deterrence and promises to replace it with a system that requires something it cannot deliver -- perfection. After all, a system that's 90 percent effective is not effective at all. If only 10 percent of Soviet missiles penetrate our defense, it's still, as they used to say in the old films, curtains. Just one missile can carry 10 warheads. As with pregnancy, there's no such thing as a little nuclear war.

We've had this sort of expectation before. We made war on poverty, and there's still poverty. We made war on drugs, and there are still drugs. The problem was neither drugs nor poverty alone, but the way people complicate things -- the way they just won't keep still.

That's the trouble with "Star Wars." Even if it were feasible, an enemy would still be facing us -- an enemy capable of coming up with its own Star Wars system and, with a little can-do of its own, ways to evade the system we have. Of course, we would do the same -- and that's just the point. Nothing will have been solved. The competition will just have been moved upstairs to space. Neither we nor the Soviets would give up nuclear weapons, anymore than a homeowner would give up a gun because he's put bars on the windows. There's always the thief with the hacksaw.

Before World War I, the doctrine of ,elan deluded the French into thinking they were stronger than they were. Now a doctrine that there is a technological answer to a political question is deluding us and, worse, complicating the chances for disarmament. Like ,elan before it, "Star Wars" is a chimera. Can- do cannot do what needs to be done.