The Reagan administration got down to serious business with the Soviets this past week - 10 months after entering the White House. The place was Geneva. The subject was nuclear weaponry in and over Europe. Can the numbers of such weapons there be reduced? The overtones on opening day were said to have been ''cordial.'' Experts assume that results, if any, might come within 12 to 18 months.
The immediate stake in the talks is the future of the North Atlantic Alliance. The United States must be seen as seeking to reduce the danger of nuclear war if the Alliance is to survive. It would begin to unravel if the Western European allies thought that Washington preferred a posture of hostility toward the Soviet Union to a search for a fair accommodation.
But it is uncertain whether the Reagan administration has entered into these talks only under pressure from its allies or because it truly seeks that ''constructive relationship with the Soviet Union'' that Secretary of State Alexander Haig Nov. 12 declared to be the administration's purpose.
The Reagan administration's attitude toward the Soviet Union has been ambiguous from the beginning.
On the same day that the talks opened in Geneva, US Defense Secretary Caspar Weinberger signed with Israeli Defense Minister Ariel Sharon a ''memorandum of understanding,'' that stated its purpose is ''to deter all threats from the Soviet Union in the region (the Middle East).''
It is customary in public documents of this kind to identify ''threats'' in impersonal terms. The Truman Doctrine, which launched the policy of containment of Soviet power, was designed specifically to keep Soviet influence, ideology, and troops out of Greece and Turkey. But the Soviet Union was not mentioned by name in the document. The stated purpose was ''to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures.''
Secretary of State Haig exchanged letters with Soviet Foreign Minister Andrei Gromyko during the first week of the administration. In the exchange both looked forward to useful relations. But four days after Mr. Haig sent his letter to Mr. Gromyko he was talking at a press conference about ''rampant international terrorism'' with an implication that Moscow was responsible for most of it. And on the following day, Jan. 29, President Reagan asserted at his first press conference that the Soviets are willing ''to commit any crime, to lie, to cheat'' to attain their purposes.
In April President Reagan opened a correspondence with Soviet leader Leonid Brezhnev, but in May he told West Point cadets that the Soviet Union is an ''evil force'' bent on destroying the US.
The opening of diplomatic business between the superpowers in Geneva was preceded by a policy statement by Secretary Haig to the Foreign Affairs Committee of the House in Washington. In it Haig said his talks with Mr. Gromyko during the UN session, ''like President Reagan's exchange of letters with President Brezhnev, were substantive and devoid of polemics and posturing on either side.''
The opening of the talks in Geneva would appear to mean that the administration is now ready to enter into business with Moscow on the theory that it could lead toward a ''constructive relationship.'' But why openly specify a ''Soviet threat'' when signing a joint memorandum with the Israelis rather than just referring to an ''outside threat''?
This raises a question whether the administration has decided what attitude to pursue toward the Soviets.
On the one hand it purports to want a ''constructive relationship.'' It has supported that by sending a team of experts to Geneva. On the other hand it continues to use words customararily exchanged between hostile countries.
The same ambiguity that characterized the US approach to the Soviets comes out in the ''memorandum of understanding'' between the US and Israeli defense ministers. The Israelis get less than they wanted and expected, but more than the Arabs considered tolerable. The Saudis are now distancing themselves from Washington on the theory that they can no longer trust the Reagan administration to support their plan for a Middle East peace.
In a similar situation with China, the US is talking about selling weapons to Taiwan in spite of Peking warnings that any such act would gravely affect their relations with the US.
In the Soviet Union, Middle East, and China, the administration continues to vacillate and avoid adoption of a clear, consistent policy approach.