Experts surveying the international security scene these days might be inclined to agree with former US Defense Secretary Harold Brown, who said recently: ''The United States is still the single most powerful nation in the world, but not powerful enough to get its way.''
Judging by reports and comments from various respected sources in the past few days, the same kind of vaguely dissatisfying and unsettling observation might be made of the Soviet Union and - more broadly - the NATO alliance and the Warsaw Pact as well.
There have been a number of new warnings of instability and potential confrontation in the world:
* In its annual Strategic Survey, released this week in London and Washington , the International Institute for Strategic Studies concluded that 1982 ''was not a very good year for the superpowers.'' Looking at increases in regional and local conflicts around the world, the respected IISS noted that although ''the United States and the Soviet Union may be potentially able to destroy each other and most of humanity . . . their might had little effect'' in reducing tensions or preventing military violence.
An ''air of mutual recrimination and bitterness'' continues between the two countries, says the London-based group, and the superpower relationship ''seems to be settling into a trough, a new pattern that will be difficult to alter. . . .''
* In a speech here Wednesday to the American Committee on East-West Accord, former ambassador to Moscow and leading Sovietologist George F. Kennan warned that US-Soviet relations are in a ''dreadful and dangerous condition,'' which includes ''the unfailing characteristics of a march toward war.''
* Speaking to Pentagon reporters recently, NATO Secretary-General Joseph Luns said this will be a ''very critical year'' for the Western alliance. Replying to those who advocate a shift in NATO doctrine to ''no first use,'' Mr. Luns said, ''We must maintain the uncertainty of whether or not the alliance will use nuclear weapons in time of war.'' Referring to public opposition to the deployment of new medium-range nuclear missiles in Europe, the NATO chief said, ''We are facing a hot summer.''
* Also this week, a group of 26 distinguished Americans and Europeans published their Report of the European Security Study. It sharply questions NATO's ability, under present circumstances, to successfully deter or defend against a potential Eastern bloc attack in Central Europe. This group says alliance countries must increase conventional defense spending goals beyond existing 3 percent per year rates of growth and take advantage of the West's technological edge to offset Warsaw Pact advantages in deployed weapons.
But most European allies have yet to meet existing NATO defense spending targets and seem unlikely to increase them to this extent, most observers agree. US Undersecretary of Defense Richard DeLauer concedes that while much of the new technology called for in this report already exists, institutional rivalries between services could make it very difficult to achieve the restructuring of forces and new form of battle management required to employ high-tech defensive weapons.
In its annual analysis of major developments over the past year, the International Institute for Strategic Studies reports that both the United States and the Soviet Union either made little progress in achieving geopolitical goals, and may in fact have lost ground. While this has exacerbated East-West problems, ''the long-term forces for moderation and compromise continue to offer a basis for hope, however slender that hope may be,'' states the report.
Among the hopeful signs noted by institute director Robert O'Neill are the continuing arms control negotiations at Geneva and signs of some flexibility on both sides. Within NATO, he says, ''Alliance leaders apparently are more considerate of each other's opinion . . . the Williamsburg summit will be a good litmus test.''
Said Dr. O'Neill: ''1983 could be a watershed year in either direction. We're not necessarily headed for a major crisis, but could have one if we don't do anything about it.''
O'Neill does not expect an early meeting between President Reagan and Soviet leader Yuri Andropov, since the condition set by Reagan for such a summit (that a clear agreement of some sort be highly probable) is not in sight.
Speaking to reporters over breakfast Thursday, Assistant Secretary of State Richard Burt also suggested that a Reagan-Andropov meeting anytime soon seems unlikely. ''I think they in principle - like we in principle - support a summit, '' he said. ''The problem always becomes the circumstances surrounding the meeting. . . . You want some expectation that you can make progress.''