As the tension and excitement mounted in Washington over the president's pending decision on the MX missile and the B1 bomber, I found myself wondering why we treat these decisions as if they were announcements of a royal wedding engagement. Or is it more as if they were the climactic moment of some sequin-beglittered television show? "And now, the envelope, please ... The winner ... The winner is ...the hardened-silo basing mode!" Other countries--some, anyway--have weapons approaching ours in cost and power to devastate, and others too must make choices concerning them. But only in America, as the late Harry Golden would have said, do we go through this recurrent ritual of buildup, suspense, rumor and, finally, with fanfare, a presidential declaration of which nuclear proposal is the lucky winner.

In this, of course, the current administration has merely followed time- honored presidential practice. Presumably because these weapons generate so much controversy and attract such fierce constituencies arguing for and against their development, the resolution of the argument about them requires, or at least invites, this sort of nuptial-announcement staging from a White House: speculation may now cease (it seems to say), this is the way it is going to be.

The first thing that strikes me about the practice is that it creates an aura, an illusion, of control where perhaps very little control exists. "I have decided this ...," a president will say. How much has he really been free to decide? How much was propelled along by its own political, technological or diplomatic momentum? Looking back on the series of such announcements I have witnessed in Washington, it occurs to me that they fall into one of two categories. About half are backings off from something a predecessor started; the other half are reluctant moves forward in response to some combination of pressures, and these are accompanied by signals that it is fervently hoped the thing won't ever come to pass.

In this latter category we have weapon systems as "bargaining chips" --for example, the Nixon Safeguard ABM and possibly the Carter 200- missile/4,600-shelter MX system that Reagan has just junked. The Carter MX system also represented a response to congressional prodding. So too, back in the late 1960s, did the Lyndon Johnson proposal of the so- called Sentinel ABM system. Johnson was trying to head off pressure to do other things.

As a matter of fact, when Richard Nixon, coming after Johnson, proposed his own revised ABM system, the so- called Safeguard, it was hailed as a step back from a more "dangerous" and elaborate Johnson administration Sentinel plan. This would be an example of the big announcement of a "step forward" that is really a backing off from a grander system. Carter did some of that with the B1 and the neutron bomb. And so has Reagan now. He has made a decision, even though it is in the tradition of the limited kinds of choice (to back off or inch ahead reluctantly) that presidents are presented with.

In refusing to buy the Carter racetrack MX system with its ambitious goal of making the American land- based missiles invulnerable to Soviet attack in the short term (closing the so-called "window of vulnerability"), he has conceded something quite important. It is not, as some of his critics insist, that he does not believe there is a danger, at least a theoretical one. It is that he believes there is nothing that can be done about it-- nothing to make those American land-based missiles invulnerable in the mid-1980s.

You do not have to buy all of Reagan's thinking on this subject to recognize something important here. It's called realism. Could it take root and flourish? I don't know. The Carter- proposed MX extravaganza represented the acme of a certain kind of problem-solving technique in Washington; unqualified faith in the power of technology and reason to come up with a "solution" to almost every challenge. But too much of it was drawing- board stuff, solutions that solved problems in theory, but which, for all their vaunted unemotional practicality, were not practical at all. To that, anyhow, Reagan has said no.

Judging from the rest of the administration's newly announced package, I don't think anyone (except maybe a few of his conservative friends on the Hill) would surmise that Ronald Reagan was about to sign up with the nuclear disarmers or loose a flock of doves in Washington Cathedral. But it seems to me significant that this most armament- minded of recent presidents has seen fit, contrary to expectation and to the desires of some of his supporters, to acknowledge a certain brutal reality concerning the limits of technology in nuclear affairs and the consequent limits of self-protection and defense.

It is the easiest thing in the world, when you live and work in the atmosphere of Washington, to lose your capacity to understand or even notice this. Years ago, when Nixon was trying to get his Safeguard ABM through the Senate, a couple of his aides came to The Post to argue the case with some of us. One of our group adopted an absolute, this-is- ridiculous stand, simply declaring that given our arsenals the argument about our peril was preposterous and that ABM sophistications were not to be taken seriously. The rest of us engaged in worried, complicated discussions with them, some a little bit for the ABM, some a little against.

Later I remember that one of the Nixon emissaries observed to me that the only way to argue successfully against them had been in the manner of our absolutist, uncompromising friend. Once we had accepted their assumptions, he said, we had to reach the same conclusions they had. I am no absolutist or ban-the-bomber in these matters, but I believe he was on to something.

What I find encouraging in the Reagan decision is the evident capacity of the president to see through some of this fog, to reject the assumptions already made that get you into the argument at a point too late to retain any freedom of thought or action. This is a fragile encouragement, but it is something. Surely it is a step toward safety and security for a leader such as Ronald Reagan to recognize the stark limits on the way the nuclear game can be played.