Every time there's a war, some people ask why the United Nations doesn't stop it. The question flatters a body that other people expect little of, but why not ask it in the second month of the Iran-Iraq war?

The United Nations looks, again, sad. The General Assembly, its rabble-rousing chamber, has averted its gaze, as it does on disputes between Third World members. Ostensibly the peace keeper, the Security Council cranked out a resolution after a week but has not so far succeeded in arranging a cease-fire or a negotiation and has sat idly by while various nations, conspicuously Jordan, thumb their noses at the resolution's call to other states not to feed the conflict.

It is an old story. Iraq attacked but Iran provoked, clouding the citing of an aggressor. This would be so even if there were the political will to press the question hard in the Security Council, which there is not, and even if council members with a veto could not shelter clients making war, as the Soviet Union surely would shelter Iraq.

The United Nations has a dirty little secret. To a lot of countries, this war, and not only this one, is not a bad little war. It is diminishing two countries regarded in their different ways as troublemakers and creating targets of political opportunity for others. So far, at least, not much outside crockery has been broken. The great powers make things dangerous when they are involved, but here they are removed -- a posture that tends to make the world safe for local wars.

What we see at the United Nations is not a display of institutional futility and helplessness, the common rap, but a display of diplomacy as usual. Countries do not abandon their sovereignty at the United Nations; they exercise it. For most of us, the inclination to be outraged by this fact dissipated years ago. It is why a country would have to be crazy or desperate to put its security in the hands of the United Nations.

But, some may ask, is this not shortsighted and risky? Is it wise in this age to let (conventional) war remain a common and usable arm of national policy? Would it not serve the American interest in global stability to give a few extra cards to the idea of peace as embodied at the United Nations -- not enough cards to even the odds, perhaps, but a few?

Jimmy Carter, early in his presidency, seemed receptive. Still, a proposal within his administration to set a sure trigger, like the movement of troops across a border, for taking a dispute to the Security Council ran afoul of the bureaucratic urge to keep tactical options open. Another possibility, to fortify the secretary general's capacity to intervene in threatening or violent circumstances, was similarly blunted.

One result is evident in the crisis performance of Secretary General Kurt Waldheim, who was chosen in post-Hammarskjold circumstances, for qualities other than individual boldness and independent stature, and who must contend with a special continuing Soviet insistence on leashing the secretary general. He can show, moreover, a time-frittering tendency to defer to other actors, in this case to would-be mediators from Pakistan and the Islamic Council. He has yet to find a resonant public voice.

So it is that Waldheim has been active in this crisis but not effective, and perhaps no one feels it more acutely than he. He has, he told me this week, successfully summoned a reluctant Security Council to address the war. By complaining directly to the Iranian president, he helped bring Iran into the debate. He has kept asking the parties to stop shooting and to accept mediation. He has offered a formula to free marooned third-party commercial ships and performed assorted humanitarian services. But the war goes on.

"I understand the uneasiness when the United Nations does not halt the fighting," Waldheim said, "but whatever is possible I've done. I draw the attention of the Security Council to the matter. But it is very difficult for me to instruct the members of the Security Council."

I asked Waldheim whether it would make a difference if he were retiring for sure next November at the end of his second five-year term as secretary general. Over the phone, he bristled:

"There is so much distortion. I have not decided whether to continue.There is no basis for saying, 'He wants to continue and his decisions are affected.' I'll decide on the basis of events leading up to next November. I would do exactly the same.I couldn't do more than I have done. We have bad public relations. The secretary general is not responsible for what the Security Council is not doing. Maybe the Security Council discusses too long before a resolution and maybe there is a question of implementation. It is unfair. I am the first to suffer under the many shortcomings of the system but it is the best we have."

Happy United Nations Day -- it's today.