London — When war breaks out in Mesopotamia, IISS phones start ringing. When an American official blurts out yet another scenario for a nuclear shot in Europe, IISS fellows are besieged by journalists seeking instant elucidation.
And when Soviet spokesmen want to impress their view of the European arms balance on West German listeners, they cite IISS figures (inaccurately, according to one recent visitor to Moscow).
IISS is an acronym to be reckoned with. It stands for the International Institute for Strategic Studies - and its name is so often coupled with the adjective ''prestigious'' in the news media that outgoing assistant director Gregory Treverton jokes that its proper title has turned into ''Prestigious International Institute.''
The IISS's reputation is indeed enviable. The institute is only two decades old and, as think tanks go, it's tiny. Yet it has already established itself as the leading authority dispensing public statistics and judgments on military might around the world. And its combination of technological, military, and political expertise repeatedly challenges popular assumptions.
The IISS was founded in 1958 - in the wake of the Suez and Hungarian disasters - by a group of academics, politicians, journalists, and churchmen who thought that Britain's Royal Institute of International Affairs wasn't doing a good enough job in focusing public policy debate on defense issues. Its first director, Alastair Buchan, immediately plunged the Institute for Strategic Studies (as it was initially called, minus the ''international'') into the thick of controversy, on British -affairs at least.
The new institute did not weigh in, however, on the single most burning issue of the 1960s: the Vietnam war. Mr. Buchan would later tell Lawrence Freedman (now head of policy studies at the Royal Institute of International Affairs) that this omission was his greatest regret about the IISS.
The IISS added ''international'' to its name in 1971, and as part of this expansion of outlook appointed the Anglo-Swiss Francois Duchene as director on Buchan's retirement. Under the present leadership of West German Christopher Bertram, IISS has maintained its international coloring. Recent fellows have included Japanese, Chinese, and Nigerian associates and interns as well as the more usual Australians, Americans, Indians, and Iranians. Its roster now boasts corporate and individual members from some 60 countries.
IISS funding is also deliberately international. Half of the (STR)460,000 (about $874,000) annual budget for its lean staff of two dozen full-time employees and up to 10 rotating research associates comes from membership fees and publication sales. The other half is contributed by private US, British, West German, Italian, and Japanese foundations. In 1978 various governments donated money to help buy the IISS's comfortably aged five-story building in the heart of London's Covent Garden theater district. But no government money is accepted for any substantive program.
Since the late 1970s the IISS has ensured a broad Atlanticist view by having an American assistant director. The caliber of the incumbents is attested to by their subsequent jobs. The first one, Richard Burt, is now US Assistant Secretary of State. His successor, Dr. Gregory F. Treverton has recently been appointed a lecturer in the public policy program at the Harvard University Kennedy School of Government. The latest assistant director is Robert Nurek, a specialist on Soviet military and political affairs with experience in the Pentagon and the US Arms Control and Disarmament Agency.
Now the institute is more outspoken about current issues than it was in the period of silence about the Vietnam war - and this forthrightness has earned criticism as well as admiration. Doves tend to regard it as a haven for out-of-power defense ministry officials. Only a few years ago, hawks attacked it for being too complacent about Soviet military superiority in Europe. (After the latter blast the IISS soon jacked up its estimate of Warsaw Pact military strength.)
In evaluating the crossfire of criticism, Massachusetts Institute of Technology professor and arms control advocate George Rathjens renders the Solomonic judgment that the IISS is indeed the home of the establishment. This embraces, however, both the uniformed military and establishment arms controllers like himself - though not the new popular peace protesters who have sprung up in Europe in the past two years.
Debate about the IISS only goes to prove just how ''established'' it is. It may be argued against, but it is never ignored. Its annual fall conferences - in recent years covering such topics as American defense, Soviet power in the 1980s , and third-world conflicts - draw many more applicants than can be admitted and still allow for the informal give-and-take that is the hallmark of the conferences.
These conferences and the annual Alastair Buchan lectures are an important launching platform for new ideas as well as a barometer of rising and falling defense consensuses. For example, West German Chancellor Helmut Schmidt's 1977 IISS lecture on the dangers of the new Soviet SS-20 missiles triggered the controversial NATO plans for new long-range nuclear weapons. And the celebrated 1979 clash between Kennedy defense official McGeorge Bundy and Georgetown Prof. Edward Luttwak over ''mutual assured destruction'' defined the most crucial debate of the late Carter years. IISS's annual publications, ''The Military Balance'' and ''Strategic Survey,'' are devoured and griped about as soon as they appear - then kept on desks the year round as standard references. The first handbook heroically assigns Army, Navy, Air Force, and individual weapons systems numbers to every country in the world with any substantial armed forces at all. (The Soviet Union currently has 30 Category 1 divisions in Eastern Europe; 67 Category 1 and 2 divisions in the European Soviet Union; 46 Category 1 and 2 divisions on the Chinese border; and perhaps six divisions in Afghanistan, the reader finds out in the 1981-1982 Military Balance.)
Strategic Survey also bravely writes crisp evaluations of the most important developments in world security in the previous year. (''The United States: Frustration and Assertiveness,'' ''The Soviet Union: Concern Over Vulnerability, '' and ''Iraq and the Divided Arab World'' are titles of chapters in the 1980- 1981 edition.)
Western defense ministries - including those intelligence divisions that monitor satellite photography of Soviet missile sites - cooperate to a considerable extent in the IISS compilation of military statistics. Warsaw Pact defense ministries do not - though the most public Soviet foreign affairs institute - The Institute for the Study of the USA and Canada maintains friendly relations with the IISS.
For their part, the Chinese, while keeping their counsel on numbers, have recently rolled out the red carpet for visits by the IISS director and assistant director - and asked lots of questions about how to found a foreign and security policy institute of their own.
Every defense ministry keeps its own numbers counts, of course, which differ in detail but probably not in scale from the IISS tally. The greatest quarrel defense professionals have with the IISS tables comes in the institute's most daring leap of all - the attempt to assign mathematical values to qualitative as well as quantitative factors in the East-West balance in Europe.
Each year the military balance assesses the number of nuclear warheads available (2,004 for the Warsaw Pact in 1981, 768 or - if US strategic subs loaned to NATO are double counted - 1,168 for NATO). Then it discounts various percentages for such factors as survivability, reliability, and penetration to come up with 872 Warsaw Pact vs. 267 or 555 NATO ''arriving warheads'' for any all-out conflagration.
For defense ministry professionals, this exercise depends so heavily on subjective assumptions and scenarios that it seems useless.
Other activities that the IISS's considerable reputation rests on include seminal work of short-term associates, their occasional Adelphi papers, and the bimonthly Survival magazine - with original articles, texts of key speeches or documents, and selective book reviews. In addition, the IISS sponsors about one book a year. The 1981 monograph, just published, promises to be the definitive work on its subject: The Evolution of Nuclear Strategy by Lawrence Freedman.
The associates come from various armed forces, foreign ministries, and universities to spend (usually) an academic year working on any project they want to. Once a week they sprint up the irregular stairs (rumor has it that there actually is an elevator in the building, though few inhabitants have ever laid eyes on it) to the informal joint discussions under the top-floor eaves. Otherwise, they cogitate in their cubbyholes and do their own thing.
This might mean making a classified targeting study for a commanding general. Or it might mean tossing out new ideas for public controversy in an Adelphi paper. The latest paper by Australian Desmond Ball, for example, contests the basic American assumption that limited nuclear war is possible; it argues instead that command and control facilities are inherently fragile and therefore would exert enormous use-'em-or-lose-'em pressure on decisionmakers in any crisis. Other recent papers have explored ''Turkey's Security Policies,'' ''A Palestinian State?'' and ''South Africa's Narrowing Security Options.''
Many of the ideas launched in this unique think tank stimulate new conceptions and new questions among security policy elites. Many inevitably fall flat. But taken as a whole, the output of the Prestigious International Institute for Strategic Studies is nothing short of indispensable.