Brussels — The Reagan administration's top diplomat now faces perhaps the most critical test of his powers of persuasion. Today Secretary of State George Shultz - on his first trip abroad since the Iran-contra dealings hit the headlines - will seek to restore Western alliance confidence in American foreign policy. He faces an audience of NATO foreign ministers still stunned by revelations surrounding secret US arms sales to Iran.
Before arriving in Brussels, Mr. Shultz said he had told European ministers at a meeting in London that the Iran affair was not a Watergate and that US policy would be unaffected. Shultz had earlier acknowledged, ``I have a rebuilding job to do.''
At a separate press conference yesterday, NATO secretary-general, Lord Peter Carrington, seemed to concur. ``I think what the Europeans will wish to hear are some words of reassurance that the administration will continue to pursue the policies that it has in those matters which affect the alliance....''
No one here expects the Iran-contra issue to be at the forefront of discussions during today's formal meeting of NATO foreign ministers, which will focus on the shape of East-West relations in the post-Reykjavik period.
But NATO officials do believe that the matter will almost certainly figure prominently in the private bilateral talks due to take place today and tomorrow between Shultz and several key European foreign ministers, including Jean-Bernard Raimond of France and Italy's Giulio Andreotti.
The NATO foreign ministers are expected to caution President Reagan, through Shultz, against yielding to Soviet attempts to exploit the administration's apparent disarray over the affair. They are particularly concerned with US-Soviet nuclear arms-control talks in Geneva.
Shultz has already moved to allay those fears in bilateral discussions he held in London earlier this week.
For Shultz, however, convincing the Europeans that the administration is still capable of mounting a coherent and credible foreign policy in the wake of the Iran debacle will not be easy.
Many Europeans have been quick to point out that the disclosures came directly on the heels of the Reykyavik, Iceland summit between Reagan and Soviet leader Mikhail Gorbachev.
In the view of some Europeans, Reagan endangered European interests at Reykjavik by nearly agreeing to an arms deal that would have left Western Europe vulnerable to Moscow.
One item to be discussed today - put on the agenda long before the Iran crisis broke - strikes at the very heart of growing European restlessness over the US speaking on behalf of the whole alliance at crucial East-West negotiations. This is the question of a new forum for conventional arms reduction talks. France wants all 35 countries involved in the recent Stockholm talks on confidence-building measures (the Conference on Security and Cooperation in Europe) to take part. But Washington wants the discussions limited to the NATO and Warsaw Pact nations.
``It's an awkward problem,'' according to the European ambassador to NATO, ``and one that the Iran affair won't help to resolve.''