If one element of good military strategy is to know your opponent, the United States could soon be at a disadvantage in its dealings with the Soviet Union. At the same time Congress and President Reagan are wrangling over how many billions of dollars to spend on national defense, the country is beginning to find itself short of experts who understand the nation's superpower rival.
A generation of US scholars on the Soviet Union - trained in the early cold-war years of the 1940s and '50s - are at or near retirement age, with few qualified replacements in sight. And even when candidates are available, some cost-conscious universities are not filling empty faculty posts.
Most observers are not calling the present situation a crisis, except in a few critical specialties. But they are raising a warning cry now because they know there is no quick solution. ''It takes longer to develop a Soviet expert than a new missile system,'' says Jonathan Sanders, assistant director at Columbia University's Harriman Institute for the Advanced Study of the Soviet Union. The problem is not one of numbers only, he says, but of quality. ''I regularly get calls from government and industry looking for a specific person. But I'm able to help only about half the time.''
According to Sen. Richard G. Lugar (R) of Indiana, who held hearings on the subject last fall, fewer than 200 young Americans are in doctoral programs preparing to study the Soviet Union. In 1968, he says, 607 US colleges and universities offered Soviet studies programs. Today, fewer than 500 do.
Efforts are under way both inside and outside government to address what many see as an alarming trend. Among them:
* Senator Lugar and Sen. Joseph R. Biden Jr. (D) of Delaware are cosponsoring a bill that would provide a $50 million endowment fund for institutions studying the Soviets.
* W. Averell Harriman, who once served as US ambassador in Moscow during his long public career, and his family have donated $11.5 million to Columbia University to fund its Russian research center.
* The Ford and Rockefeller Foundations, once large and steady contributors to Soviet studies which later left the field, have offered modest new grants of $2. 5 million and $2 million, respectively.
Yet many observers say that although these and others efforts begin to spotlight the problem, they are not far-reaching enough.
The efforts are important, says Alexander Rabinowitch of the Russian and East European Institute at Indiana University in Bloomington. ''But they're all policy oriented. It's terribly important and in our national interest to be concerned about Russian studies broadly - the humanities, the culture.''
Any new efforts to support Soviet studies will probably never compare with the ''almost heroic period'' of the '40s and '50s, says Helmut Sonnenfeldt, a guest scholar at the Brookings Institution. Although shock waves from the success of Sputnik and the strong interest of the Kennedy administration continued the boom into the '60s, he says, ''then China, Africa, petroleum economics, other issues started to become more interesting.'' For many years, he says, ''We were able to live off the fat of the great numbers who'd earlier poured out of these institutions. Now, unless some trends are reversed, we will have serious problems.''
The problem today, says Daniel Matuszewski, who heads the Soviet program for the International Research and Exchange Board in New York, is that ''concern over the presumed weakness of our military is not matched by worries over our analytical capabilities.'' The board recently issued three studies on the problem, including one that shows the estimated number of new Soviet scholars falling far short of what's needed.
That contrasts sharply with the Soviet effort to study the US. A Rand Corporation study, not yet released, shows a large Soviet commitment - from graduating specialists at prestigious institutes in Moscow down to drilling schoolchildren on world geography.
Comparing the Soviet and US efforts is ''like the Yankees vs. a little league team in numbers, effort, and money,'' offers Columbia's Sanders. ''For example, I talked to one Soviet expert who could tell me all about the primary election laws in Iowa.''