Washington — ''The trilateral countries and the Soviet Union stand at a crossroads. They can either reach accommodations which will make possible a reduction of their military competition or face an increasingly unstable world in which the economic burdens of defense will grow and the security of all nations will diminish.''
This warning, in a report published this week by the Trilateral Commission, comes as East and West stand poised to deploy new, more threatening and destabilizing weapons.
Looking at the state of the world today, this group of world authorities says that the security of Western Europe, Japan, and North America ''will be much less divisible than in the past,'' that cooperation among them ''will be more important to their security in the 1980s than in previous decades.''
Like the problems it describes, the report's recommendations are complex and in some cases at least superficially paradoxical.
For example, it opposes deployment of the MX intercontinental missile. But it says the nuclear freeze could be dangerous and concludes that the NATO decision to deploy new intermediate-range missiles in Europe beginning in December must not be reversed or delayed.
It urges NATO to reduce its arsenal of battlefield nuclear weapons, but says the Western alliance (at least for now) should not renounce the first use of nuclear weapons to halt a conventional attack from the Warsaw Pact. And the commission report calls for substantial increases in alliance spending for conventional forces.
Recognizing that economic revitalization is essential to national security as well, one of the report's authors, former US arms control negotiator and Ambassador Gerard Smith, says, ''There's no doubt that it could be a painful process.''
But the alternative to a strengthened conventional deterrent, Mr. Smith told reporters in Washington, is a further slide toward superpower confrontation and possible nuclear war.
Founded 10 years ago, the Trilateral Commission is a private group of political leaders, bankers, business executives, and academics from the three regions.
Prominent Americans affiliated with the organization include former Secretary of State Henry A. Kissinger, former national security adviser Zbigniew Brzezinski, former chairman of the President's Council of Economic Advisers Alan Greenspan, former Defense secretaries Robert S. McNamara and Harold Brown, and former US Ambassador to the Soviet Union George F. Kennan.
Members of the Reagan administration previously associated with the commission include Vice-President George Bush, Defense Secretary Caspar W. Weinberger, and Ambassadors William E. Brock III and Arthur F. Burns.
Federal Reserve Board chairman Paul A. Volcker also has been a member.
While the report's authors take exception to some Reagan administration positions and policies, the differences are not fundamental.
For example, the report states: ''The heart of trilateral security will continue to rest indefinitely on strong survivable nuclear forces. This fact must not be underestimated or deprecated.''
But Mr. Smith, who held key positions in Republican and Democratic administrations and is now president of the Arms Control Association, also noted a recent shift in Reagan administration action and rhetoric. He cited a more moderate position on integrating arms control and strategic modernization, and said, ''We're pleased to see that.''
The lengthy report is the commission's first in-depth effort in the areas of national and collective security. Most of its earlier commentary and recommendations - which, although unofficial in status, are closely read in Japan and the Western democracies - have dealt with world economic issues.
The report, citing ''the dangers that the Soviet Union presents,'' foresees ''a greater need for consultation and action on a trilateral basis,'' and says commitment to NATO and the US-Japan Mutual Security Treaty ''must be reaffirmed to our populations.''
It urges the US, Japan, West Germany, Britain, Italy, Canada, and France, - the seven summit nations - to broaden their collective concerns to include security improvements.
The report, titled ''Defense & Arms Control Policies in the 1980s,'' was written by Gerard Smith, Paolo Vittorelli (Chairman of the Italian Institute for Defense Studies and Research), and Kiichi Saeki (Chairman of the Nomura Research Institute).