Three and a half years ago, under U.S. pressure (and hoping to force the abandonment of the U.S. Strategic Defense Initiative), Mikhail Gorbachev promised President Reagan he would reduce his menacing force of SS-18 heavy missiles by 50 percent.

Two weeks ago, Jim Baker went to Moscow and, in effect, gave those missiles back. Baker called it "progress."

Here's what happened: Ronald Reagan returned from the Iceland summit with Gorbachev's heavy missile concession in 1986. Recently, however, the Soviets have started to produce a new type of SS-18 that is twice as capable as the old one. The still uncompleted draft START treaty that Baker was negotiating in Moscow should freeze the Soviets with the old SS-18. This would give meaning, effect and permanence to Gorbachev's 1986 pledge.

But in Moscow Gorbachev insisted -- and Baker agreed -- that he be allowed to replace old SS-18s with new ones. As each new SS-18 is equal to two old ones, Baker's concession effectively vitiates the 50 percent cut.

It's reverse magic: now you don't see it. . . now you do.

Giving back the heavies was not the only concession Baker made during his ministerial meeting in Moscow. Further concessions included a range limit on cruise missiles that significantly favors the Soviets and a reversal of American opposition to including unverifiable sea-launched cruise missiles in a treaty-related undertaking that Baker has sought to justify as merely "politically" binding.

But the Moscow concessions are not the most inauspicious clouds darkening this week's summit. That distinction is reserved for the issues on which no "progress" was made in Moscow: Soviet withdrawal of its unwanted occupying troops from Eastern Europe; agreement to a unified Germany within NATO; an end to the suffocating blockade of Lithuania; movement toward a sharp reduction in conventional military forces and a start on the demilitarization of the Soviet economy.

Taken together, these issues define the American agenda for the U.S.-U.S.S.R. relationship, including the Washington summit. That they have taken a back seat to the Soviet agenda -- George Bush's signature on a "statement of principles" for a treaty that would limit only modestly the Soviet strategic nuclear forces that concern us most -- is a worrisome sign of misplaced priorities and maladroit tactics.

The United States has long resisted "statements of principles" concerning complex treaty issues. Moscow tried repeatedly to get Ronald Reagan to sign up to such statements, the effect of which is to diminish Washington's insistence on the crucial details which, like a genetic code, defined the growth potential for future military forces affected by a START treaty. In a draft treaty that already exceeds 700 pages of impenetrable prose, the details are the essence. Get them wrong, as we did with the first SALT agreement in 1972, and back doors we can't even imagine open the way to developments the treaty is meant to seal shut.

As for the American agenda, prompt Soviet agreement to the withdrawal of the 380,000-strong Red Army from Germany would allow the future security framework for Europe to be negotiated without the darkening of Soviet military might. Think of it as 380,000 points of light.

With its economic power growing in a security vacuum, a unified Germany relegated to neutrality or disintegrated from NATO would eat away at the foundation of the "common European home" that has been the leitmotif of Gorbachev's European diplomacy. Think of it as millions of termites.

Independence for Lithuania would spare Gorbachev a costly, protracted -- and ultimately unavailing -- repression. It is wrong to think that Gorbachev can't let go of Lithuania because doing so would hasten the unraveling of Moscow's empire. That unraveling began when the failure of Marxism-Leninism could no longer be denied and when the illegitimacy of the Hitler-Stalin pact was acknowledged by the Soviet leadership. Throttling it with a brutal and intimidating blockade is no solution. A tough anti-secession policy will only radicalize independence movements from the Baltics to Azerbaijan.

In all the Soviet Union, Lithuania is probably best positioned to implement Gorbachev perestroika. Lithuania could light the way for Soviet economic reform in much the way that Hong Kong once demonstrated the benefits that might follow from Chinese liberalization and decentralization -- and from a window on the Western economic order. Think of it as a test of wisdom, not of wills.

The West would benefit from an agreement sharply reducing Soviet conventional military forces and military budgets. But Gorbachev would benefit even more. There's little doubt that Western forces and budgets are being reduced, even though we could afford to keep them up if we wished to do so. With a quarter of the Soviet Union's meager national income going to the military, and much of that to maintain a huge conventional force, the prospect of successfully relaunching Gorbachev's stalled economic reform is negligible.

The militarization of its economy has helped bring the Soviet Union to the brink of insolvency. The shops may be empty, but the arsenals are full. The United States is now deciding the size of the American defense budget. Seldom have the interests of East and West converged as clearly as they do now in bringing to fruition an agreement cutting conventional forces and military budgets.

The most helpful thing President Bush can do for President Gorbachev this week is to insist on the American agenda at the summit.

Think of it as a new beginning, the end of an error.

The writer was an assistant secretary of defense in the Reagan administration.