At the start, the wags had seen something fitting in the choice of Venice for this week's summit meeting: a bruised alliance coming to a city once-believed sinking to prove that it too still stood on solid ground.

In fact, the West's major leaders demonstrated their solidarity here with suspicious ease.

On political matters, they reaffirmed united opposition to the Soviet invasion of Afghanistan but avoided discussion of specific countermeasures. On the world's economic problems, they pledged new oil-saving targets through the development of alternative energy production means, but otherwise outlined only general concern for the usual trouble areas.

Somewhat surprisingly, senior U.S. officials in briefings here said they had not been sure, despite months of preparation and consultation, whether they would get even this much agreement from their major allies, particularly on Afghanistan -- a revealing sign of the nervous tension in the alliance.

As far as it went, the summit succeeded at least in showing the Europeans and Japanese seeming to fall into step with the United States. The appearance of unity, if not the deep substance, prevails.

The Soviets may even have helped in this respect with the announcement that they were withdrawing from Afghanistan. Knowing michief when they saw it, the Western leaders were determined especially at this moment to make clear they could not be divided by Kremlin scheming.

The worry now is what will happen when the government chiefs return home and once again put varying interpretations on Soviet moves.

The summit statement on Afghanistan declared the current Soviet occupation "unacceptable" and spoke of the "courageous resistance" of the Afghan people -- all familiar language from past communiques. Of special significance, however, was the additional statement that the invasion represents not just a regional threat but a broad danger to world peace.

This has been the American view from the start. But the West Europeans, particularly the French and West Germans, have tended to view occupation more as a local affair that should not upset their developing relations with Moscow and Soviet-dominated Eastern Europe.

While acceding here to America's strategic view, the allies offered no further measures that might pressure the Soviets to pull out completely from Afghanistan. In closing remarks at a press conference, only President Carter among the seven leaders even mentioned Afghanistan.

West German Chancellor Helmut Schmidt mentioned his intention still to visit Moscow next Monday for two days of talks with President Leonid Brezhnev. He said that while he would hold to the western line on security, he will be speaking first for the interests of West Germany.

How to maintain unity in the West while recognizing differing national interests has become the central dilemma.

The alliance was never meant to be a committee. It is an association led by the United States and despite their resistance lately to U.S. initiatives, European diplomats say they would welcome the return of strong, consistent American leadership.

But a lack of confidence among European leaders in the Carter administration's handling of foreign policy has led to more action being considered within narrow national interests.

Never very comfortable with U.S. "insistence on the need to punish" the Soviets for invading Afghanistan, some European have called for the drafting of a global Western strategy that would emphasize a "division of labor" among the allies and would aim chiefly at strengthening Western influence in Southwest Asia and the Mideast.

But the details of any such strategy never emerged at this summit. U.S. officials believe it is not possible to draw up a sort of master plan for Western foreign policy in a world of today's complexities.

This leaves critical differences outstanding between Europe and the United States -- on the Soviet Union (how to respond to Afghanistan), on China (the degree to which Peking should be played against Moscow), on the Middle East (how to quickly produce peace), and on North-South relations (where to find a common line for assisting developing countries).

The point of this summit was to signal that the Western alliance is in much better shape than all the talk and reporting have suggested. Against the dreamy seascape of Venice, seemingly floating between water and sky, even Carter and Schmidt claimed their differences had vanished.

It also helped that the main items of business were economic rather than political. As Canadian Prime Minister Pierre Elliott Trudeau observed, the West seems to do a better job cooperating on its economic problems than it does on it political ones.

Indeed, one justification for such showy occasions of mass summitry is to give Western leaders a boost in pushing through unpopular policies at home. If Americans can see that the Europeans and Japanese face the same concerns about inflation and the energy crunch, the president gains the advantage of an international alibi in selling his own painful energy program.

After persuading his partners to set the ambitious goal of reduction in oil consumption during the next decade equivalent to 15 million barrels a day, Carter declared, "We will now return to our own countries to ask for more sacrifices."

With three of the seven leaders -- Carter, Schmidt and French President Valery Giscard d'Estaing -- all soon facing national elections, such displays of unity in economic adversity matter all the more.

Something else of note seemed to emerge at this summit. Running through official statements was a clearer acknowledgment by the West's leaders of the limits on them to manage the world in the way they once confidently thought they could.

The transfer of economic power to the oil-producing countries and the desire for power by the developing countries have together eroded the former attitude of Western invincibility. There is a strong sense now of the need for the West to enlist more actively the help of the Arab world in particular in aiding the poor and stabilizing the world's monetary system.

British Prime Minister Margaret Thatcher referred to this when she reportedly told the other leaders that the West needed "to get alongside the new sources of power." While no one quite knew what she meant, the comment captured the sense of tilt in the world balance felt on this side of the Atlantic.

The threat to the Western leaders was once very obvious and direct. Everyone knew it came from the communist East and everyone knew what was to be done about it. That made the task of rallying the Europeans behind the United States a relatively simple one.

But today, Moscow moves more indirectly, invading regions far removed geographically from Europe. In general, the threats to the West have multiplied, becoming more varied and more difficult to manage with a single team approach.

The European governments think they are more intensely aware of these shifts and of the implications for diplomatic alliances than is the self-centered United States. In any case, the need for common political action relates to all.