The question of whether our allies are bearing their fair share of the Western defense burden has triggered an important foreign-policy debate in the United States, with voices being heard from both ends of the political spectrum. This is a welcome development because the burden-sharing issue must sooner or later be addressed if public support in the United States for our system of alliances is to remain strong.

Unfortunately, however, the sober debate that this question deserves is being threatened by demagogic rhetoric and half-baked proposals whose only effect will be to confuse the issue and ultimately weaken our alliances.

Donald Trump recently published an impassioned full-page newspaper ad that reported that our friends around the world were "laughing" at us and that the time had come for them to "pay for the protection we extend as allies."

Rep. Pat Schroeder takes this frustration one step further with her ill-conceived proposal to impose tariffs on imports from Allied countries that fail to match the percentage of the gross national product that the United States spends on defense.

Schroeder's proposal linking trade and security seems more a thinly veiled attempt to gain greater support for protectionist policies than a serious effort to stimulate greater Allied defense spending. More to the point, Schroeder's concept is flawed by its attempt to equate burden sharing with actual defense spending. From my vantage point as an American representative in Western Europe, there are three key points that must be kept in mind if the burden-sharing issue is to be examined from a serious and constructive standpoint:

First, while our European allies certainly can and should spend more, their contribution to the common defense has been steadily growing for the past decade and a half while ours has fluctuated dangerously. Since 1970, in fact, non-American NATO defense spending has grown by 35 percent (after inflation), while over the same period the non-U.S. proportion of total NATO defense expenditures has risen from about 23 percent to more than 30 percent.

What this means in concrete terms is that if war were to break out in Europe today, the great majority of the manpower and equipment engaged on the NATO side would be European -- some 90 percent of the ground forces, 75 percent of the air power and 50 percent of the naval assets.

Second, burden sharing cannot only be understood in terms of aggregate military spending. For instance, most of our European allies continue to maintain a peacetime military draft. Conscription results in substantially lower manpower costs than would be the case if the Europeans relied on all-volunteer forces, and proportionately higher expenditures on equipment and readiness training. Would Schroeder agree that we should reintroduce the draft if the Europeans match our defense spending levels? I doubt it.

Finally, and perhaps most important, any burden-sharing calculus cannot ignore the less tangible but nonetheless very real burdens imposed on some of our allies by the concentration of military forces and activities on their soil. The Federal Republic of Germany offers a very good example: no larger than Schroeder's home state of Colorado, but with a population in the range of 60 million, West Germany hosts 400,000 troops from six other nations in addition to maintaining a standing force of its own of 500,000 men.

Each year, nearly 5,000 military exercises are held throughout West Germany -- more than in any other Allied country -- while the air forces of seven nations fly more than half a million sorties annually in German air space, many at treetop altitudes. Moreover, the West German government provides some 4,000 military installations and training areas for the use of Allied forces at no cost.

In assessing the relative burdens of Western defense, we also need to remember that the Germans, together with the British, the Italians, the Belgians and the Dutch, have gone ahead with the deployment of new intermediate-range missiles -- including in the German case both mobile Pershing II and cruise missiles -- despite Soviet political blackmail and terrific domestic opposition. This stands in marked contrast to the United States, where after more than a decade of debate we still have not deployed a single mobile MX missile.

One must question whether American voters would be willing to accept a concentration of military forces and activity similar to that readily tolerated by the West Germans and other Europeans. The populist politics of protectionism and ally-bashing might play well in Colorado, but is Schroeder prepared to ask her constituents to make the same kind of nonfinancial sacrifices for defense that the Germans and many other Europeans have made? Again, I doubt it.

Make no mistake: our principal allies do need to spend more for defense. As for proposals for linking trade and security issues, these must be rejected as unworkable and counterproductive. Any such steps not only would lead to a trade war, and thus weaken our economy, but would drive us and our allies further apart at a time when we need to be working more closely together to strengthen our common security. The writer is U.S. ambassador to West Germany.