Stockholm — The Western allies - including the United States - are taking the high ground going into the Stockholm ''confidence building'' conference that opens today. This time around it is the West that is speaking peace and the Soviet Union that is adopting a shrill tone. That's quite a switch from the rhetorical positions of the two superpowers over the past three years - and it does wonders for allied unity.
The new Western position presumes that momentum is now on the side of the West. NATO's Euromissile decision of 1979 is finally being implemented, and deployments clearly will not be blocked either by European peace movements or by opposition parties like the West German Social Democrats.
Under these circumstances President Reagan feels - to the immense relief of his European allies - that he has stick in hand and can therefore afford to speak softly. This at least is how NATO allies here saw the President's nationwide address Monday.
In it, he cited US military power as essential ''to negotiate successfully and protect our interests.'' But Mr. Reagan called for ''constructive cooperation'' and dialogue with Moscow. ''Our challenge is peaceful. . . . We do not threaten the Soviet Union,'' he stated, stressing ''common interests'' between ordinary Soviet and American citizens and their governments, foremost among them ''to avoid war and reduce the level of arms.''
''We have a long way to go but we are determined to try and try again,'' he added. ''We may have to start in small ways, but start we must.''
In the past three years, by contrast, Mr. Reagan's very loud exhortations to his countrymen to be ready to fight a nuclear war with the Soviet ''evil empire, '' if need be, profoundly frightened Europeans living in the potential nuclear battlefield.
His belligerence - as it was perceived here - fueled the growth of the European antinuclear and sometime anti-American movements more than any other factor, according to both peace activists and government officials. It also made more difficult the NATO governments' task of selling the Euromissile deployments to voters.
With Mr. Reagan's election-year change of heart this week the allies are clearly pulling together again in East-West relations. Everybody is eager to be conciliatory (while still proceeding with the Euromissile deployments).
West Germany, of course, has beavered away consistently to preserve some East-West dialogue over the past three years. Specifically, it has worked to persuade a reluctant Washington to agree to the present 35-nation Stockholm European security conference. Canadian Prime Minister Pierre Trudeau has for months been pushing his own plan to bring the five nuclear nations to an arms control conference.
The French and British are also speaking softly. Paris is projecting a thaw in its bilateral relations with Moscow. And British Prime Minister Margaret Thatcher, whom the Soviets once dubbed ''the Iron Lady,'' is inspiring press speculation that she might be a possible ''bridge'' between East and West when she visits Hungary a few weeks hence on her first official trip to the Soviet bloc.
Moscow is not amused by all this Western bonhomie. It keeps trying instead to restore a mood of crisis. The apparent aim is to convince European public opinion that the current NATO missile deployments are bringing the drums of war closer.
But it is hard for the Soviet doomsayers to grab the headlines when an amiable Reagan is radiating cooperation, or at least ''peaceful competition.'' In the new atmosphere, the news media attempt by chief Soviet Euromissile negotiator Yuli Kvitsinski to blame Washington rather than Moscow for the year-end Soviet walkout from the Euromissile and strategic arms control talks is falling flat.
For the European man in the street the images of the two superpowers are thus suddenly being reversed. Now it is the Reagan charm that is pitted against the Gromyko scowl.
Before, it was ominous talk in Washington about ''protracted'' and ''limited'' nuclear war that was pitted against soothing Kremlin offers of nuclear freezes. And a lot of Europeans - opinion polls show - concluded then that the US was trigger-happy and at least as great a threat to peace as the Soviet Union.
With Reagan's Jan. 16 speech this image has begun to change - to the enormous relief of West European governments, which expect their publics to return to a healthier view of the US as Europe's protector rather than the provoker of the Soviet bear. And with the US shift they believe 1984 is going to prove a lot less turbulent for allied relations than 1981, 1982, or even 1983.