President Reagan's "Star Wars" plan is aimed at Soviet nuclear missiles, but if poorly handled it could score a direct hit on the political fortunes of this nation's European allies.

So far, the U.S. administration has been surprisingly ineffective in addressing the concerns of major European allies. A lack of definition and consensus in U.S. sales language on Star Wars, formally known as the Strategic Defense Initiative (SDI), has caused trouble among Europeans. And the lack of forethought in such initiatives as Secretary of Defense Caspar W. Weinberger's "60-day" offer last week of technological partnership -- which many interpreted as an ultimatum to the allies -- has compounded the problem.

These conclusions arise from two days of public and private discussion here among 120 prominent West Germans and 80 Americans, including a number of senior government officials and parliamentarians, at the 13th biennial American-German conference, sponsored by the Atlantik-Bruecke (Bridge) and the American Council on Germany.

Although the agenda for the conference ranged widely, Reagan's Star Wars plan dominated the political, military and at times even the economic discussions. So encompassing was the interest that Harvard Prof. Gregory F. Treverton, in summing up a series of private meetings related to NATO, said, "I have the feeling that if we'd had a conference here in Dallas on pediatrics, it would have turned out to be on SDI."

On the surface, the West Germans and other European allies are going along with Star Wars, as shown by last week's unanimous approval of "research" in this pursuit by the NATO defense ministers meeting in Luxembourg. Just beneath the surface, however, German officials and political figures are deeply concerned about what SDI means for their security, their relations with the Soviet Union and their internal politics.

In the face of European concern, calls for intensified U.S. diplomatic efforts came from two former senior U.S. officials, William G. Hyland and Lawrence S. Eagleburger.

Hyland, noting there was "no public preparation, no political preparation, no consultation with allies and no game plan about how to proceed" in the early stages of Reagan's proposal, said in an interview Star Wars "has the makings of a major alliance crisis unless handled carefully, perhaps with some U.S. concessions."

Hyland, editor of Foreign Affairs magazine and a former CIA and National Security Council official, suggested formation of a high-level Atlantic alliance group on how to accommodate SDI, at least for the rest of Reagan's term.

Eagleburger, who retired in May as undersecretary of state for political affairs, recommended in an address that the administration begin diplomatic discussions centered on the French and British, whose nuclear arsenals are "clearly threatened" by the defense plan.

Weinberger's efforts to coordinate with the European allies last week won no plaudits from German officials and politicians here, some of whom expressed puzzlement and irritation at his offer last Tuesday to permit European nations to participate in Star Wars research if they respond within 60 days.

The West German political opposition, which was represented here, immediately accused Weinberger of laying down a 60-day "ultimatum" and treating Germany as "a colony" of the United States.

German government officials involved in hammering out the response to Weinberger said their study will take longer than 60 days, leaving the impression that they would lose face by responding within this U.S. "deadline."

U.S. officials said Weinberger's deadline was not discussed in advance with the State Department or National Security Council, which were taken by surprise and considered it provocative and unwise.

Official comments also suggested a lack of clarity within the government about the essence of Weinberger's offer, notably how far the United States is willing to go in transferring the valuable fruits of its research to the Europeans and other potential SDI partners.

Weinberger, speaking to reporters on his way home from Europe Saturday, said "there wasn't anything rigid" about his 60-day deadline. "The only purpose was to show that we are ready to and are in fact proceeding with a lot of research" and are eager to have European participation, he said.

The revival of concern about alliance solidarity comes as Germans and Americans still are congratulating themselves over successful deployment of U.S. Pershing II missiles despite powerful opposition from West German peace groups and the Soviet Union.

Following the first deployments, the peace groups and political allies were left exhausted and without an issue. Star Wars, which was proposed by Reagan in a speech two years ago but only recently came to the fore internationally, is renewing the energy of the opposition.

The very existence of West Germany on the border of the Iron Curtain, and especially West Berlin in the midst of East Germany, has depended for decades on the credible threat of U.S. and allied military response, including massive nuclear retaliation.

Reagan and some of his aides, in appeals for SDI, have raised doubts about the long-term viability of deterrence through the threat of retaliation -- sometimes called Mutual Assured Destruction -- and at times have suggested it is immoral.

The degrading of deterrence is "one of the most difficult problems of the years to come," said a West German official. Noting that previously the West German peace movement, rather than the U.S. ally, was attacking the morality of nuclear weaponry, the official added, "I think it is a mistake by the U.S. government to moralize the question."