Le Grand Tour

This is the weekend when Ronald Reagan begins the first European Grand Tour of his presidency and begins also to enjoy the reward for having adjusted his foreign policies to the needs and the interests of his colleagues in the NATO alliance.

The reward will come in the form of many minutes of priceless prime-time exposure on American television every evening of the tour. He will appear as the friend and confidant of a Queen in England, of one of history's most respected Popes in Rome, of the President of France and of the Chancellor of West Germany. He will appear as the statesman president, reforging the alliance and strengthening the machinery for the containment of Soviet power.

The ground has been prepared. The tour comes after he agreed to back the British in the Falklands, over the protest of his pro-Argentine ambassador to the United Nations.

It comes after he agreed to renew strategic arms talks with the Soviets, and arranged for such talks to begin on June 29, in spite of the disapproval of many of his original conservative backers.

And it comes after he agreed to give up his earlier efforts to block the building of the pipeline that will bring natural gas from Soviet Siberia to Western Europe.

If only he could also promise an early decline in American interest rates, the tour would become an exercise in almost perfect harmony marred only by the inevitable antinuclear demonstrations. But even these will be less vehement and less difficult for police control now that Mr. Reagan is on his way to arms limitation talks with the men from Moscow.

The preliminaries for the tour have exposed the substantial changes that have occurred in Mr. Reagan's foreign policy postures over the 16 months since he entered the White House.

In an interview with two Time magazine correspondents during the trip's planning stage, he stated his conviction that the NATO alliance is not just America ''generously helping our allies.'' ''It's mutual,'' he declared, ''as much a defense line for us as it is for them.''

That is a big change from a year ago when White House officials were muttering about ''going it alone.'' There was even talk about abandoning the NATO alliance.

All that has been pushed aside. The reason and the explanation for the change came in another passage from the same interview. Mr. Reagan said:

''. . . until you are where a President sits and have access to all the information he has, you can't really criticize. You can't know the reason for some of the moves that have been made.''

Before he sat ''where a President sits,'' Mr. Reagan criticized most freely a foreign policy deed carried out by his predecessors including Messrs. Nixon and Ford as well as Carter. But now he sees things differently. He can begin to understand why they seek a nuclear arms treaty with the Soviets, why the Germans are reluctant to get into an economic cold war with Moscow, why the British felt they had to take firm action against Argentina, and why the French are particularly unhappy about high US interest rates.

Perhaps the biggest thing of all he has learned is that the NATO alliance is as important to the US as it is to the allies in Western Europe.

All of which means that it is now possible for Mr. Reagan to visit the other leaders of the alliance in Europe as an accepted member of the club.

That was not possible when he first met them in Ottawa during July of last year. The aides to the great men at that meeting managed to stitch together a communique at the end that made polite sounds. But as soon as the various people got back to their own capitals, the facts leaked out that to the Germans, French , British, and Canadians, Mr. Reagan was still in the k'ndergarten stage of foreign policy understanding.

There was no meeting of minds at that first ''summit'' of the Reagan administration. There were only preliminary efforts to try to make Mr. Reagan understand something of what the world is really all about.

All of which adds up to the main world news event of the week; the news that the President of the United States can now talk intelligently and intelligibly with the leaders of the other members of the NATO alliance. Communication has been restored after the hiatus which tends to come with every change of administration in Washington.

Thus the grand tour has some substance. While no new business of importance can be done during the brief time available, the fact of Mr. Reagan's cordial reception symbolizes and expresses a considerable strengthening of the fabric of the NATO alliance of recent date. It is no longer in danger of being abandoned by Washington. There will be easier and more fruitful communications between Washington and the other NATO capitals in the aftermath.

Other main events of the current week are clarifications of the military situation in both the Falkland Islands war and the Iran-Iraq war.

In the first the British achieved a spectacular success in building a beachhead on the far side of East Falkland from the Argentine invasion force, in breaking out of the beachhead, and in encircling the Argentines in a narrowing perimeter. It was all done in 10 days.

In the second the Iranians imposed decisive defeat on the Iraqi invaders of Iran. Iranian soil was largely cleared. The only remaining question is when and how far the Iranians will carry an invasion of their own into Iraq.

In both of these wars those who began them must be wishing they had never done it. The aggressors are suffering humiliating and costly defeats. Potential other aggressors should heed the warning.