Sam Nunn may know more about national defense than any member of the Senate, but in urging an open-ended policy of "patience, " he contributes little to the national debate. As a policy for the Persian Gulf, patience has its virtues. Patience also has its limits.

As chairman of the Armed Services Committee, Nunn has been able to put on quite a show in the past few days. One after another, his witnesses have made the same case: Let us give sanctions several years, if need be, to do their work. Nunn summoned two former chairmen of the Joint Chiefs of Staff to support a waiting game. James R. Schlesinger, a former secretary of defense, testified to the same effect.Let us pray over this policy. Saddam Hussein, dictator of Iraq, pulled off his blitzkrieg conquest of Kuwait on Aug. 2. The United Nations at once voted for economic sanctions. By Aug. 10, virtually the whole world had fallen in line. We are now well into December. Under the latest U.N. resolution, no military action may be taken until Jan. 15 at the earliest. One is inclined to ask with Isaiah, Lord, how long?

Nunn's argument has the sound of reason: "If we have a war," he says, "we're never going to know whether the sanctions would have worked." True enough, but if we wait for the sanctions to work, we may wait forever. Time is not on the side of the allied coalition. Time is on the side of Saddam Hussein.

This is true, or so it seems to me, for reasons that have more to do with human nature than with military strategy. What has been the world's recent experience with economic sanctions? May we recall the example of what used to be Rhodesia? When its government stubbornly refused to yield to black rule, everyone ganged up behind sanctions against Rhodesia.

A year or so later, I happened to be in what used to be Salisbury. The sanctions had not worked. Every conceivable kind of consumer goods had found its way around the embargo. The government was having no serious problem in keeping its planes flying and its troops supplied. Call it greed, or call it opportunism, or call it what you will, it was business as usual in Rhodesia.

At the moment, South Africa is under all kinds of sanctions. Have these impositions done their work? Have the restrictions brought South Africa to its knees? The United States bars commerce with Cuba. Does anyone seriously believe that U.S. goods have become unattainable in Havana? The lamp of experience should guide our steps. Viewed in that light, why should we believe that sanctions will work against Iraq?

Other considerations challenge the wisdom of waiting for a few years. The international coalition formed against Saddam Hussein looks fine on paper, but paper fades. So will this fragile coalition begin to lose its forceful image. No exhortations will sustain the morale of troops bogged down month after tedious month in an inhospitable desert. Here at home, political support is bound to dwindle. The American people have little patience with patience.

"I don't believe in taking needless risks," Nunn says. No one can argue with that proposition. But if a risk is simply a risk, and not a needless risk, we have a different calculus. The senator adds, gratuitously, "I believe any president has to think about whether there are other alternatives before he risks a large number of American lives."

Merciful heavens! Does Nunn really suppose that President Bush has not thought about the alternatives? Bush knows war. He has been there. He is not an impetuous man, given to impulsive and ill-considered actions. The terrible, fateful responsibility of sending young men to die weighs upon him night and day.

But this situation cannot be permitted to drift along indefinitely. The credibility of the United Nations, never very high, is at stake. Of greater importance, the credibility of the United States is at stake. Bush repeatedly has called upon Saddam to withdraw "immediately and unconditionally" from Kuwait. At some point, such calls cease to be impressive and become ludicrous instead.

The key consideration may be stated in three words: now or later. If Saddam's naked aggression is not nullified - if the dictator is not disarmed now, if our troops are anticlimactically brought home - nothing will have been gained and much will have been lost. Yes, of course, let us be patient. But when patience wears unbearably thin, we must strike.