ALTHOUGH IT MAY seem of little comfort to Americans -- and certainly not to the people of Poland in this exceedingly tense moment -- the Soviet Union today is probably nearing the limits of its considerable abilities to effectively use and expand its military power.

Short of another vast increase in its commitment to defense, something akin to mobilization for all-out war, the Kremlin is going to feel the mounting strain of maintaining its security at home and imposing its will elsewhere. From the Chinese frontier to the western border with NATO, from near our own Alaska to the depths of Afghanistan, Moscow has vast military obligations that are growing steadily more complex.

Moreover, the Soviets' expeditionary ambitions are expensive. They are largely underwriting Cuba and Vietnam, the Hessians of the communist world whose roles in Africa and Southeast Asia wholly reflect Moscow's interests. And now the Kremlin itself has dispatched at least token military units to 11 Third World countries, although these remain a fraction of the forces stationed inside Eastern Europe.

For the sake of comparison, at the height of American military involvement around the world in the mid-1960s, the United States had resources for fighting major wars in Europe and the Far East with a smaller conflict in Cuba. That is roughly the Soviet situation today: They are prepared for serious conflicts on the Chinese and European fronts and are engaged in a nasty war with Afghanistan rebels. To do more, the Soviets would have to strip their present defenses -- which they dare not do -- or substantially enlare their defense budget, which is already sapping the country's civilian sectors.

With about 65 percent of the United States' gross national product, the Soviets have been spending about 11 to 14 percent of their GNP annually on defense needs, according to the CIA, contrasted with about 5 percent of GNP spent by the United States. "This has contributed to maintaining a ceiling on Soviet economic growth," Abraham Becker, a Rand Corporation economist, observed earlier this year, "and perhaps even to its retardation."

The case for the coming restraints on Soviet military capacities is only a theory, mind you, not an absolute assertion of fact. Divining Soviet intentions and measuring the relative strengths of the United States and U.S.S.R. is the stuff of theological disputes among strategic analysts in this age of multiple nuclear overkill. Ultimately, the real question is not who is stronger, but who would be prepared to use what is undisputably the gigantic power of our respective nuclear arsenals.

We don't know when the Kremlin would blink any more than we can be sure of our own limits. So when we talk about what Moscow can and would do, it is with the assumption that the Kremlin will be run by rational people intent -- as the United States is also -- on fashioning a world as amenable to its interests as possible. If one accepts that the Kremlin is greedy for influence but not pathologically focused on world domination, then it is easier to understand why their military commitments may be nearing, or already have reached, the point of overload.

Until a year ago, the debate in the United States about Soviet plans divided generally between those who felt that Moscow's military buildup was precautionary or reactive rather than offensive and those who perceived a major Soviet global threat at American expense. The invasion of Afghanistan ended the discussion, because it was now clear that the Soviets would use their amassed power directly and brutally if they had to outside their previous domain. The current menacing encirclement of Poland merely underscores the Soviets' sinister motives when their interests are endangered.

Still, the fact remains that mean as they are, the Soviets have good -- very good -- reasons to be worried about their ability to maintain their security. Imagine for a few moments sitting in the Kremlin and surveying the scene.

Start with China. In 1967, the Soviets kept 25 divisions along the border with its enormous and fundamentally hostile neighbor. Today, there are 46 Soviet divisions stationed there. The Chinese are a nuclear power. In addition, they have assiduously courted the United States in the past two years and the Carter administration, at least, was embarked on a strategic relationship with Peking -- meaning the sale of quasi-military goods and technology of the kind no American would ever dream of suggesting should be supplied to the Soviets. The Chinese have an ideological antipathy to the Soviets. As for the Russians, on a daily basis, they are far more worried about a Chinese invasion than trouble from the West.

Next, Afghanistan, where the Soviets have dispatched six divisions -- 85,000 men -- to quell, so far without success, the insurgency which a year ago threatend the Moscow-backed regime. The Soviets, the record shows, are undeterred by the niceties of waging an unpopular war as the United States tended to be in Vietnam. So crushing the Afghan revolt by whatever means available is possible. But the effort has not proven easy thus far and the history of outside involvement in Afghanistan is that the locals go on fighting until the intruders are exhausted.

In Iran, which also adjoins the U.S.S.R., the political chaos represents both an opportunity and a problem. The Soviets may well be able to exploit the upheavals in Iran to their own ends and come, in time, to neutralize a weak and divided neighbor. For this moment, however, the Ayatollah Khomeini and his mullahs are almost as rabidly anti-Soviet as they are anti-American.

The Soviets are playing an extremely intricate game in the region, backing the Iraqis in their war with Iran while trying to woo the Iranians at the same time; and sidling closer to the Syrians although Syria remains a sworn enemy of Iraq. As the world can clearly see, the Middle East is a labyrinth and the Soviets are badly trapped in it.

There are about 50 million Moslems in the U.S.S.R. -- the peoples of Uzbekistan, Kazakhstan, Tadjikistan and so on. Few outsiders, particularly westerners, can claim to understand the impact of the fundamentalist Islamic revolution in Iran on these Soviet brethren. But it is a fair bet that the Moscow leadership would have preferred that the religious revivalism hadn't happened so close to home.

On to Europe. The NATO allies unquestionably have their difficulties. The Europeans grudgingly accepted the American lead on sanctions against the Soviets after Afghanistan. Yet there are some important psychological -- and practical -- factors that often get overlooked when considering how Europe looks to the Russians.

First was the NATO decision in the fall of 1979 to go ahead with the deployment of theater nuclear forces -- medium-range missiles -- in West Germany. The notion that missiles capable of hitting and destroying cities such as Moscow and Leningrad will be based on the soil of Russia's traditional German adversaries sends shivers down the spines of those who remember, most recently, the Nazi invasion of 1941.

To deter NATO ground forces (and keeps its Warsaw Pact allies in line), the Soviets maintain 30 divisions in Eastern Europe, up four from 1967. The International Institute for Strategic Studies' annual report on the military balance, 1980-1981, relects the general consensus that Warsaw Pact strength has been growing faster than that of NATO, but it also observes drily of the pact, "there must be doubts about the reliability of some of its members and the value of their forces." If one thing is certain about the current Polish crisis, it is that Moscow would be foolish to count on the Polish army to carry out its objectives.

Summing up the view, then, the Soviets are surrounded by avowedly unfriendly forces, and matters, in this year of Poland, Iran, Afghanistan and U.S.-Chinese detente, have gotten more disturbing, not less. Add to this the fact that the climate in the United States is readier for an extensive expansion of American power than at any time since the early 1950s and the Kremlin has its hands very full indeed.

Can the Soviets match the U.S. popular willingness to add a trillion dollars to defense spending over the next five years? Suppose Poland is invaded, with all the tumult and heightened world tensions that would follow -- would the Soviets be able to accept additional heavy burdens as well, say in the Persian Gulf or Africa? My guess is they would not.

Aside from the huge expenditures already necessary to keep Cuba and Vietnam afloat in appropriately militaristic style, if Poland were suppressed, the Soviets would assume another enormous financial handicap because the Polish economy would almost certainly come to a standstill. And Poland is a land of 35 million people. To understand the size of the Cuban obligation, for instance, consider that the $8 million a day that Moscow spends is substantially more than all current U.S. bilateral foreign economic aid, aside from what we put out through such institutions as the World Bank.

There is, of course, an argument to be made that the Soviets will go on matching and exceeding American defense spending and expanding their military posture. That has been the case in the past whatever the cost in money, manpower and sacrifice. As Adam Ulam of Harvard University writes in the current Commentary magazine:

"In general, the notion that 'the Russians have their own troubles' has been a deceptive solace to many Americans as they comtemplate our own not so splendid condition and the lengthening shadow of the U.S.S.R. in the world . . . . Who, what, it might be asked, will compel the U.S.S.R. to turn inward, unless there is a power strong and determined enough to make Soviet foreign adventurism too risky and expensive? In the absence of such power, is it not more likely that the Soviets will try to compenasate for and to obscure their internal weaknesses by aggressive policies abroad?"

Perhaps.

But to underestimate the size of Soviet burdens now and overstate what capacity Moscow has for further expansion is, to my mind, as much a mistake as complency in the face of Soviet power. The Soviet Union is unquestionably a big, strong and unyielding adversary. But to believe that it has no restraints on what it is able and willing to do in the world means that inevitably the United States and U.S.S.R. will confront the terrible moment when we have to stop them once and for all or risk our own immediate destruction. Wherever we can find them, we must search for reasonable alternatives to that choice.