PARIS -- The armada the United States has assembled for Persian Gulf duty has served a modest but useful purpose. It has persuaded American friends and adversaries abroad that the Reagan administration is still in business.

To note this is to thank heaven for small favors, in the hope of encouraging more and larger blessings.

Now comes the hard part. Having demonstrated that it can still wield power despite the Iran-contra affair, the Reagan presidency must show that it can wield power effectively. The original mishaps and accidental nature of U.S. policy in the gulf did not encourage confidence on this score.

The U.S. warships have served as a kind of policy bludgeon, focusing the attention not only of Iran but also of America's European allies, the Soviet Union and the perpetually apprehensive Arab states of the gulf on U.S. power.

But these are temporary and fragile gains. It would be dangerous to mistake them for enduring cooperation or acquiesence. The reality is that the United States is more alone in the gulf than it appears.

Five European countries have finally agreed to send their own warships to the gulf area, but only because of intense diplomatic pressure from Washington.

The pressure worked, but officials in Britain, France, the Netherlands, Belgium and Italy now wonder how America will extricate itself -- and them -- from the Iran-Iraq cross fire.

The Europeans are practicing their own version of a "flexible response" strategy on the United States in the gulf. They are being as vague about how they would react to military escalation there as the United States is about its eventual nuclear response to a Soviet invasion of Europe.

The Europeans have insistently kept their ships under national command, remaining independent of U.S. logistical support and operational decisions. Obviously, they do not want a repeat of the disaster that joining the Reagan administration in Lebanon brought them four years ago.

"It simply wasn't worth continuing to fight the United States on this," explains a senior official in one of the five governments. Adds an adviser to another leader who agreed reluctantly to go along with the American request not only to send ships but also to publicize the intention to do so:

"So far so good. But how long do we have to stay out there? What is it that will get the Americans out? We don't get answers when we ask Washington this."

Moreover, West Germany and Japan have declined to associate themselves with the American effort in the gulf. There is mounting concern in allied capitals that Bonn is seeking a separate peace with Tehran, in order to get Iranian help in freeing the one remaining West German hostage in Beirut and to protect Germany's privileged trading position in Iran.

West Germany thus joins the Soviet Union and China as question marks in the U.N. Security Council vote on mandatory sanctions against Iran that the United States will seek now that the U.N. effort to get a gulf cease-fire has failed.

The Soviets have been made aware that Washington sees the vote as a test of the new "pragmatism" that Moscow has been advertising in its Middle East policies. A cooperative attitude on the gulf in the U.N. vote would be an element in fashioning a successful summit later this year.

More fundamental to the White House calculation must be a sense that the Soviets see the U.N. process as the best way to reduce the troubling American military buildup, which is "precisely the circumstance that Gorbachev's Middle Eastern initiatives aim to prevent," as Israeli scholar Galia Golan notes in the current issue of Foreign Affairs magazine.

If this analysis is correct, the Soviets will not try to broker a solution to the Iran-Iraq war on their own. Nor will they maneuver to dig the U.S. deeper into a dangerous pit of its own making.

They would, instead, cash in the chips they have been accumulating in the gulf in the interests of the larger, emerging objective of supplying the United States with reasons to limit or reduce its military presence abroad.

Such supply-side diplomacy by the Russians obviously carries long-term risks for American strategy. But the White House has little choice left but to come to a clear U.S.-Soviet understanding on the next steps in the gulf. Perhaps the best thing that can be said for U.S. policy at this stage is that it has forced the same realization on Moscow.