The Post's editorial on the DIVAD gun, "A Choice of Weapons," (Aug. 19) requires both a clarification and a correction.

The clarification concerns the basis on which the Defense Department will decide whether or not to continue to buy the DIVAD. This decision was held up and expenditures on the program suspended last year so that the system could be tested and the arguments about it addressed in light of the test results. This admittedly conventional approach to the issue seemed like a good idea to most people when it was adopted, and it's a good idea today.

The Post now proposes, however, that the arguments for and against continuing with the system not be settled on their merits. "DIVAD has become a symbol," it solemnly concludes, as if nobody knows who makes symbols in this town. Arguments about the merits of the system are troublesome. Whatever the Army may need to provide air defense to its troops, it is argued, Secretary Weinberger should kill DIVAD to salvage the credibility of the Pentagon's procurement process.

During World War I, some officers of the French army briefly adopted the practice of shooting every tenth poilu in units displaying insufficient ,elan. This practice of "decimation" served, in the view of those who employed it, "pour encourager les autres" and to enhance their own credibility. In fact, the policy was inhumane, anti-rational and diminished troop morale and command effectivensss. The Post's new procurement policy of killing the occasional weapon system for symbolic effect is similarly flawed.

The credibility of the Pentagon's procurement process will not be established and maintained over the long term by evading the merits of issues, but by facing them. Nor will our troops be properly equipped to defend either themselves or the country if symbolism determines what weapons we will and will not buy, instead of a careful analysis of the threats we face and the options available to reduce them.

The clarification: The Defense Department will, despite The Post's advice to the contrary, decide whether or not to continue to buy DIVAD on the basis of the test results and the merits of the arguments for the different courses available. Such an approach would seem unremarkable in normal times; the wonder is that it does not commend itself to The Post.

The correction involves a more serious issue. The editorial raises the question of why the Pentagon is being forced to consider cancelling weapons programs such as DIVAD at this time. Actually, "raises the question" is not quite accurate; the editorial flatly answers the question. Since, however, the answer given is wrong, the question should be explicitly set out.

Why isit that weapons systems are under budget pressures today? The Post says it's because "the buildup was underpriced." "The defense budgets now projected," it confidently continues, "will not buy all the weapons the planners earlier said they would." This assertion is wrong both as a general proposition and as it applies to DIVAD.

In the case of DIVAD, as the editorial states later, suspension of the program was the result of doubts about the system's effectiveness and the desire to have further tests before continuing with the buy. What of the more general issue?

In 1981 and to a lesser extent in later years, the assertion was regularly made by critics of the administration's defense program that it was underpriced -- i.e., that the budgets then projected would not buy all the weapons the department said they would. Over the years, as the evidence came in, this criticism was completely discredited.

Only The Post's mindless word-processor keeps sending it back to us like last year's erroneous utility bill whenever the defense budget is the topic and it may advance an argument. It is regrettable that, unlike the electric company, The Post consistently fails to include a notice to the effect that if you really understand the issue you should disregard the point.

This argument has become such a source of embarrassment to those who originally developed it that it wasn't even mentioned when Rep. Les Aspin, chairman of the House Armed Services Commmittee, announced a couple of months ago that the Defense Department had actually overestimated the cost of its program in its early budgets. Aspin pointed out that substantial funds anticipated to be required had either not been spent, had been reprogrammed, or had not been appropriated at all. This is correct.

It is vitally important that the public understand that the reason defense programs are being reduced this year from what was previously planned -- and they are -- is not tha cost estimates have risen but that "the defense budgets now projected," as The Post puts it, have come down from what was previously projected. In fact, as everyone knows, the defense budgets now projected are lower by many billions of dollars than originally proposed by the president.

These budgets will not buy the same defense program as the larger ones. This should surprise no one. Far from confounding the predictions of the administration, the secretary of defense has consistently anticipated this development and stated that lower budgets would mean stretching some programs and canceling others. Some people dismissed these statements as alarmist; many chose to ignore them. They embody, nonetheless, a certain inevitable logic, and are being borne out by events.

As we in the Defense Department go about adjusting our programs to the lower levels agreed to in the congressional budget resolution, there should not be the slightest doubt as to why this task has become necessary. Too many people have worked too hard for too long to cut the defense budget in recent years. Many other people have opposed them. Now that the first group has succeeded in significant measure is not the time to suggest that this fight has been irrelevant or without consequences.

It has had consequences of the greatest importance for our security. It has reduced our ability to fund needed defense programs in the immediate future. But let's be clear about one thing: the budget pressure that is genuinely affecting defense programs is not the result of some misrepresentation by the Defense Department about the cost of its programs years ago, as The Post assumes. There was no such misrepresentation. The pressure stems directly from the lower budget levels agreed to by Congress earlier this month.