The decision by Iranian militants to show the world an alleged "secret" document that they said had been purloined from files in the occupied U.S. Embassy adds an ominous new factor in the battle of American intelligence against Soviet forgeries aimed at discrediting the United States.

Whether the militants have what they claim to have or whether the alleged CIA assignments for the two new staffers at the U.S. Embassy in Tehran are bogus, the surfacing of the document compounds the problem of indentifying and exposing proliferating Soviet forgeries. These forgeries are now known to have drawn both President Carter and Vice President Walter Mondale into their worldwide operations.

The Soviet forgery game was analyzed early this year in a classified government document called "the forgery offensive," which opened with this flat assertion: the dangerous Soviet game of lying about the United States in the struggle between the two super-powers is undergoing "an appreciable upsurge."

"The political purpose of these forgeries, their technical sophistication and intelligence reporting all point to the Soviet Union, its various East European allies and Cuba as being the responsible parties,"the document said. e

The study containing that charge against Moscow was followed in late summer by a second analysis, limited to "official use only" and published by the Defense Intelligence Agency -- a major branch of the U.S. intelligence community. It proclaimed that Moscow has "continually employed forged documents to implement foreign policy, support political objectives and to lend substance, credibility and authenticity to their propaganda claims."

The United States has never played the forgeries game against Russia or any other country. One reason could be that in an open society forgeries would almost surely be exposed by those opposing the practice -- by politicians, for example, who in the past have taken pride in exposing undercover operations by the CIA, regardless of foreign policy objectives.

The Soviets have a closed society and no known scruples against dirty tricks of any kind. But in the efforts -- described as being "of suspected Soviet origin" -- to put false words in the mouths of the president and vice president of the United States touched a new low. The falsification of Jimmy Carter's spoken word came in December 1977, in the form of a bogus press release from the United States Information Agency (now the International Communications Agency). It purported to be a verbatim report on a speech Carter gave in the "american perspective series."

Newspapers in Greece -- and almost certainly in other countries where the forgery never surfaced -- received the phony Carter speech in the mail. Two newspapers in Athens published it. In his "speech,"Carter flayed the Greeks for letting down NATO, demanded far higher defense spending by Greece and made demeaning remarks about this major Mediterranean ally.

The forgery involving Mondale came just over a year ago when Xeroxed copies of an interview he allegedly gave to a European newspaperman named "Karl Douglas" were mailed to Paris-based correspondents of several newspapers.

In the "interview," the vice president cast aspersions on Egyptian President Anwar Sadat and Israeli Prime Minister Menachem Begin. Mondale, according to the bogus "interview," called Sadat not the master of his own house (implying the then-pending treaty with Israel would not be adhered to) and claimed that Begin was suffering from a "terminal illness."

Both these efforts were crude, and neither one did American policy much, if any, damage. But they illustrate this point; there is no limit to the Soviet effort to "disinform" governments and peoples of the world about the perfidy of the United States by exploiting all techniques of forgery and black propaganda. Moreover, other attempts to undermine the United States have had conspicuous success.

In 1978, in an altered version of a genuine State Department document known as "Airgram A8950," dated Dec. 3, 1974, U.S. embassies in Europe were ordered to collect information "on ways to bribe European officials and to develope other covert means by which to damage or eliminate foreign trade competition" with the United States. The timing was calculated to cash in on the uproar in the United States over bribery accusations against U.S. corporations.

This forgery, American intelligence now believes, was "an eminent Soviet forgery success" despite some sloppy discrepancies such as bad punctuation in the covering letter that came with fuzzy copies of the alleged airgram.

With superpower competition now heating up, partly under the stress of the Iran crisis, top intelligence officials have ordered the anti-forgery watch put on overtime duty. But for every forgery discovered, there probably are half a dozen that go undiscovered. The whole world is a forgery market and it is inconceivable that the United States will not be damaged in the days of heated rivalry that lie ahead with an adversary who plays by only one rule: the rule to win.