Except as a measuring rod, the summit meeting on industrialized nations in Venice this weekend shapes up as a nothing event. For the measuring rod shows a weakening of solidarity between the United States and its principal allies since the summit meeting in Tokyo a year ago. Far from reaching a term, moreover, the unraveling process proceeds apace.

At Tokyo last year, President Carter could count on two unquestioning supporters. The death of Prime Minister Masayoshi Ohira in Tokyo two weeks ago removed one of them. With him there goes the long rule of elder statesman mindful above all things of the American role in Japan's postwar miracle. There now takes over in Tokyo a new generation -- far less beholden to Washington.

Joe Clark of Canada is the other. Fidelity to the American connection was one of the reasons for his defeat in the election last February. Pierre Elliott Turdeau, who both preceded and succeeded Clark in Ottawa, does not have to be taught the lesson. He aligns his country with the United States only when it can be made to pay. His pressing need now is to strike a deal with dissident provincial leaders. The means putting Canada first even at the expense of the American connection.

In Europe, geography, economic success and population make West Germany this country's most important ally by far. Chancellor Helmut Schmidt has never been an admirer of Jimmy Carter, and his doubts and suspicions have been sharpened by the American performance of Iran and Afghanistan. Schmidt believes that the Carter administration is quite capable -- for frivolous reasons of American internal politics -- of taking actions that could shatter his national and political interest in tolerable ties with Russia and East Germany.

To preserve his Ostpolitik, or Eastern policy, Schmidt arranged to meet with the Soviet leader, Leonid Brezhnev, in Moscow at the end of this month. Carter, a week ago, sent Schmidt a private letter warning him not to compromise plans for modernizing the NATO missile force on his Moscow visit. That letter -- which was leaked in circumstances adverse to Schmidt and which was surely not necessary in view of the chance for private talks in Venice -- can only worsen relations between the two men. It is now a genuine question whether they can work out even the minimal elements of a concerted stategy for Schmidt's Moscow visit.

France, for both reasons of state and of internal politics, cannot easily let the West Germans take the lead in Europe. So President Valery Giscard d'Estaing has been pushing French primacy in all the usual ways.

He beat Schmidt to the punch by having a meeting with Brezhnev in Warsaw last month. At a European summit meeting in Venice last week, he blocked the nomination of a serious figure as the new head of the European Commission, or Common Market. At the same meeting, he pushed through a "European initiative" supposed to supplement the U.S. role in forecasting a settlement between Israel and the Arabs.

Prime Minister Margaret Thatcher undoubtedly means to ally Britain with the United States. But she has doled out foreign policy to languid aristos in the Tory party who share the Old World contempt for this country's populist foreign policy. So despite Mrs. Thatcher, Britain has swung away from the United States and joined in the French move to cut European deals with Russia and in the Middle East.

The Italian government of Prime Minister Francesco Cossiga has apparently been strengthened by a weakening of the Communists in the municipal elections a fortnight ago. That means it will insist all the more forcefully on Italian participation in any general discussion regarding allied policy toward Russia and the Middle East. Since the French make it a point of pride only to talk about such agusut matters with the Americans, British and Germans, the louder Italian voice will have the paradoxical result of making a general meeting of minds in Venice even less likely.

The United States, in the past, could absorb these pretty little squabbles in some larger design. But if Carter ever had such a version, he has now lost control of its basic elements. Economic policy in this country has been taken over by the Federal Reserve Board. Congress, as its rejection of the administration's plan for an oil import fee shows, owns energy policy.

In the circumstances, no serious American initiative for allied cooperation is feasible until after the election this fall. It will be something of a success if Carter puts Venice behind him without doing still further damage to the ties that bind this country and its allies.