When Uncle Sam reaches into your pocket to help pay the cost of defense each year, he extracts about $520 for every man, woman and child in the United States.

But in Canada -- this country's friendly, prosperous neighbor and partner in the 15-nation North Atlantic Treaty Organization (NATO) -- the government spends only $157 per person on defense.

In Denmark, another NATO member with one of the world's highest living standards, the per capita defense expenditure is $303. In Great Britain, it is $314 per capita.

In Japan, defense costs citizens of one of the world's most successful economics only $87 apiece.

Yet each of these countries has a stake in the survival of the West, and each benefits from the protection of the Western alliance generally and the predominant role of the United States in particular.

So why is the burden of defense costs shared so unequally?

Finding an answer to that question has become especially important these days, because the twin crises in Iran and Afghanistan have once again focused global attention on defense and global fears on the possibility of war. f

In the Persian Gulf, the region rattled by upheaval in Iran and the Soviet thrust into Afghanistan, it is all the West's oil that is threatened, not just that oil imported by the United States.

These same dual crises and the question of how to deal with them also have heightened tensions between the United States and many of its allies. Thus, the notion that allies may be shirking their common defense duties could cause a shift in public opinion here in the long run when it comes to future decisions about keeping U.S. troops abroad or making commitments to the defense of Western Europe and Asia.

The answer, however, is complicated. Statistics don't tell the whole story nor do they necessarily lead to a conclusion that all the allies are shirking.

For example, ending the draft and substituting a higher-paid all-volunteer force has helped boost the Pentagon's manpower costs to 54 percent of this year's proposed $142.7 billion total outlay for defense.

But aside from the United States, England and Canada, the rest of the major NATO allies -- West Germany, France, Italy, Holland, Belgium, Turkey, Greece, Portugal, Norway and Denmark -- continue to rely on a draft to fill the ranks.

This allows some countries, such as France, to field sizeable ground armies with less of a dent in their overall budget and allows others, such as Turkey, to field much larger armies than they could otherwise afford.

Similarly, only England and France among the U.S. allies have nuclear weapons. The other allies thus do not have to pay for a vast array of missiles, for submarines and bombers to deliver them, or for the research needed to develop new ones.

Given the size and resources of the United States and the overwhelming position of strength with which it emerged from World War II, the United States also has been the only individual counterweight to the Soviet Union and thus the only country that has, for the past 35 years, also retained an ability to intervene militarily almost anywhere in the world, something that also costs money.

Indeed, there are probably few governments of any ilk in Europe or Asia that would like to see a vast expansion of either German or Japanese military strength, especially in ground armies, yet those two countries are in the best position economically to do so.

According to the latest statistics published by NATO, the major European military powers -- England, France and West Germany -- come closest to the United States in relative defense spending when measured as a percentage of each country's gross domestic, or national, product (GDP).

In 1979, the United States spent an estimated 5.2 percent of its GDP on defense. England spent 4.9 percent, France 4.0 and West Germany either 3.3 or 4.1, depending on whether expenses for West Berlin are counted.

Of the smaller countries, Portugal led with 3.8 percent of GDP, with Belgium and Holland at 3.3 percent. Norway at 3.2, Italy at 2.3 and tiny Luxembourg with 1 percent. No 1979 NATO statistics are available for Greece and Turkey, two relatively poor countries with serious problems of their own. London's respected International Institute for Strategic Studies (IISS) estimates that both spend more than 4 percent of their GDP on defense, largely to combat each other's forces.

Of the major countries in NATO, Canada is, by any measure, the clearest laggard -- spending only 1.8 percent of GDP, according to NATO, and $157 per head, according to IISS.

The lack of a greater Canadian contribution is felt in other ways too, because many military leaders say privately that the 3,000 Canadian brigade based in Europe is, man-for-man, the best fighting unit on the continent.

In terms of dollars spent per person on defense, the IISS estimates that in 1979 the Germans paid $396, the Belgians paid $363 apiece, the French $349, Norwegians $347, Netherlands $338, Danes $303 and the Italians, among the poorer countries, $124.

Though Japan is not a member of NATO, Tokyo clearly is part of a western alliance in its broadest terms. It also is the major industrial power whose economy and people have benefited most from being virtually free of the burdens of non-productive defense spending in the post-war era while benefiting from the protective power of the United States. p

While the United States turned West Germany into a major military power, it never pushed Japan in that direction.

Japan, according to IISS, spends less than 1 percent (0.9) of its GDP on defense, equal to about $87 for each of the 116 million or so Japanese on that crowded island.

Japan, however, also provides a good example of the need to look beyond statistics to understand the potentially volatile question of who is and isn't carrying their share of the load.

Because of Japan's huge GNP -- $1.1 trillion in 1979 -- the $10 billion spent on defense last year makes Tokyo the eighth largest defense spender in the world, according to IISS, behind the Soviet Union, the United States, China, West Germany, France, England and Saudi Arabia.

Because countries such as Japan and West Germany have higher industrial productivity these days than the United States, they also get "more bank for the buck" out of their equipment purchases.

While virtually all U.S. specialists in the field believe Japan needs to spend more, many argue that attention should be focused more on how a country spends its defense dollars, in terms of the overall effectiveness and contribution to joint defense, rather than so heavily on the percent of GDP figures.

Japan, for example. While the U.S. Seventh Fleet in the Pacific has been put under great strain by the Indian Ocean crises and has declined in strength, the Japanese Navy already has taken on added importance.

Japan now has 45 destroyers in Asian waters, triple the number of comparable U.S. ships. It has more submarines than the United States in the region, a large fleet of patrol planes and the world's largest, next to the Soviets, fleet of minesweepers.

Japan's constitution, molded by the postwar U.S. occupiers, contains the famous "no war" clause. But this has been interpreted more liberally to a point where Japan's "self defense force" now numbers some 250,000 personnel, including a 155,000-man army. Its land forces, however, which are most expensive, and Japan's land forces are least likely to expand because of the undoubted concern in China or Korea if they did so. Thus, no big percentage increase in spending is likely to show up.

While the Carter administration is now leaning heavily on Japan to increase its defense contributions, specialists believe it should be in the Air Force and Navy, where Japan could play the most effective role. The White House hopes Japan will boost its defense share of GDP over 1 percent in the next few years. But the Tokyo government of prime minister Ohira fell last week, in part over his acquiescence to the U.S. pressure.

Specialists say Japan actually has increased its defense spending over the past 10 years by almost 8 percent annually, but wide fluctuations in the dollar/yen exchange rate and in inflation make statistical comparisons difficult.

In recent years -- and especially since the Afghan crisis -- as the Carter administration has done something of a turn-around on the need for greater defense preparedness in view of the Soviet build-up, the United States has leaned hard on the European allies in NATO to increase their military spending in real terms -- that is beyond inflation -- in the region of 3 percent annually.

The administration has had some, but not complete, success in this campaign.

Last year, the United States, Belgium, France, Holland, Norway, Portugal and England met the goal. This year, Germany has said it will make it, as will France, England and the United States. Italy, striving to play a larger role in big power politics and to be cooperative in defense undertakings, may also make it.

Denmark is the biggest problem. The other smaller countries, however, are all borderline. In these countries, the problem has always been a feeling within the population that war in Europe is highly unlikely and, if it comes, the contribution of Holland or Denmark might make western defense against the Warsaw Pact last only a day or two longer than it could without them.

The United States sharply rejects such thinking because it is also used by smaller countries to avoid paying their share of common defense programs. The Belgian refusal thus far to join in paying for the fleet of airborne early warning radar planes is one example irritating Washington.

The sudden involvement of the United States in the Persian Gulf, however, has demonstrated dramatically that the arguments could be changing; that U.S. forces aimed for 30 years at reinforcing Europe and Asia may not all be there in the future, and other countries had better think more seriously about providing more for their own defense and about helping the United States elsewhere. Whether this message will get across remains to be seen. It could easily fade in the glow of a new Soviet "peace offensive."

German, French and British forces already provide the bulk of the air and ground forces defending Europe. But the United States wants are Europeans to beef up their dangerously low stockpiles of the latest ammunition, to increase their ability to airlift U.S. troops in an emergency and to get their own reserves in shape.

Bonn's 495,000-man armed forces provide the largest and most well-equipped fighting force in Western Europe. They are being modernized with 1,800 new tanks and scores of new jet warplanes.

France, though outside the military sector of the NATO alliance since 1966 (when Charles de Gaulle withdrew French forces), remains within the political alliance and it is widely assumed that French troops would join with the other allies in an emergency.

France maintains just over 500,000 men under arms and its arsenal of nuclear-tipped missiles, bombers and submarines makes it the world's third largest atomic power.

Furthermore, French President Valery Giscard d'Estaing has recently pointed out that France is the only western power that has steadily increased the proportion of defense spending in the national budget and GDP over the past five years.

France also plays a unique role in other ways in that it has shown itself willing to use military forces to intervene in Africa. The United States has backed away from that while privately supporting the French efforts.

Both the French and the British maintain sizeable navies. Experience, in the case of England, and an actual military presence in the case of France, in the volatile Indian Ocean region could also be helpful in a crisis there.

England, whose 323,000-man military traditionally has been the best trained yet worst equipped forces in NATO, is moving to substantially step up defense spending. The defense "white paper" published last month forecasts a real increase of 3.5 percent next year, including modernization of England's four nuclear missile-firing submarines.

Indeed, while Soviet military spending has outstripped that of the United States in recent years and is responsible for the new push to increase western preparedness, the Pentagon's own statistics show that in terms of alliances, military spending by NATO actually exceeded that of the six-nation, Soviet-led Warsaw Pact in the decade of the 1970s and, if the goal of 3 percent real growth is met, is projected in stay ahead.