Washington last week viewed its own proceedings with astonishment. Matters of military defense have long been subject here to tedious, windy arguments that seemed, when they were over, to leave things pretty much as they were. This changed. From the production and deployment of the MX missile to the overall size of the military budget to the conduct of Pentagon contractors, familiar arguments over defense preoccupied the Capital. Only this time, something happened. The usual side didn't win.

I think we are not just in the presence of another of those periodic bursts of enthusiasm for some issue which everyone (a)pronounces to be of life-and-death consequence and (b)almost immediately drops because it has become unfashionable (it's not only people who are subject to Andy Warhol's rule about celebrity only lasting 15 minutes; the same is true of issues like poverty, bombs and busing). What happened last week was that the military mystique, for the first time, failed to protect those who invoked it. And once impregnable fortresses of argument and assertion fell.

It is true that various administrations have lost individual tests of will with Congress before over weapons systems and other military matters. And it is also true that there is still plenty of money around to pay for defense. But there was a sense last week in which everyone knew that the residual control the Pentagon and the president had always exercised over the scope and even the ultimate outcome of these arguments had vanished. The situation was outside their powers of manipulation. They had to deal.

Why was it happening? Why now? Secretary of Defense Caspar Weinberger has complained that there is an injustice here. He points out that his own department initiated many of the crackdowns on contractors that are providing the evidence currently being used to punish the Pentagon and justify cuts in its budget. This has a certain validity in the life-isn't-fair department. But Weinberger is merely discovering in Congress what Khrushchev discovered in Hungary: conceding a little bit of anything can be dangerous. You do not control what happens next, and you do not necessarily appease demands for reform when you introduce a few; more likely you create a whole new sense of dissatisfaction and possibility. In any case, it has not been the Pentagon's reasonableness, its efforts at improvement and reform in matters such as these that has brought on its troubles. On the contrary, it has been an impression of unreasonableness and rigidity that brought it to its present unaccustomed pass.

This is what you will hear from many of its traditional supporters on the Hill and elsewhere in Washinton. Far from being too accommodating, the Reagan administration is seen by them as having been unaccommodating to the point of arrogance. The early postelection insistence on high defense expenditures and no tax increase of any kind, a hardening of position as they saw it, struck many key prodefense Republican senators especially as perilous and preposterous. Those who wanted to help, but who also wanted to move the administration somewhat, report they were at best ignored, at worst fought bitterly. The position taken by Defense spokesmen lacked credibility, and this finally was the key to it: the arguments made in behalf of the administration position were not believed by those who had been prepared to take them seriously. And the unyielding, defiant pursuit of such arguments, in turn, struck them as contemptuous and overweening.

Speaking of Contemptuous and Overweening (surely a Dickensian law firm), the performance of some of the contractors is relevant here. Perhaps it is understandable that folks who had gotten away with so much over the years simply could not believe, especially in the age of a defense buildup, that anyone had either the instinct, the clout or the staying power to take them on. It was, after all, thought positively antic a few years ago when Reagan's Navy secretary, John Lehman, presumed to file a counterclaim against General Dynamics for causing the Pentagon to lose money. Some of these contractors stand in the same relation to the government that big borrowers do to big banks, beyond being punished for a failure to deliver fully and on time. But the scandals and actual corruption that were revealed in a seemingly unending flow this year finally registered with the public and reinforced the growing displeasure being expressed by onetime Pentagon helpers on the Hill.

Some commentators have accurately pointed out that the most vivid horror stories -- the $748 pliers, the $435 claw hammer, the kennel fee for boarding a contractor's dog -- do not really go to the much larger and harder-to-deal-with aspects of waste in the procurement process. But, as with the equally marginal matter of Mrs. Nixon's gift- earrings in the Watergate drama, these symbols sometimes galvanize people on an issue in a way that the intricacies never can. They are among the accidents that worsened the administration's position. So too, in a way, I suppose is the absence of John Tower from the Senate this spring, a circumstance widely cited as contributing to the outcome, since he was particularly skilled at dealing with and for the administration on this subject. But basically what has happened is not an accident: this effort to make the American military enterprise subject to rational decision making and controls was overdue.

It is good news, though its outcome will not necessarily be good. For this is just a beginning. No one who has given more than 30 seconds' thought to the monstrosity of present procurement policy making and regulation will suppose that reforms coming out of Congress will automatically make things better. And no one who has watched many of the Pentagon's more severe critics over the years can be confident that they will be wise in victory. Until now they have had the luxury of knowing their complaints would not be acted on; much of what they have said has been mindless and reckless in its simple antidefense attitude. But after this spring's events it will no longer do to be against virtually everything that is not made in one's own district. Are they really serious about maintaining and strengthening legitimate defense while getting rid of waste? We will find out soon. What comes next is the hard part.