President Reagan and Soviet leader Mikhail Gorbachev are scheduled to meet at 1:45 p.m. today in the East Room of the White House to sign an arms control treaty that was considered wildly improbable by the very people who set in motion the negotiations that produced it.

The treaty, which eliminates U.S. and Soviet intermediate-range and shorter-range nuclear missiles, reverses a long, slow buildup of nuclear arms in Europe that was kicked off by Soviets in 1977 when they deployed SS20 multiple-warhead nuclear missiles within range of Western Europe.

As a result of the intermediate-range nuclear forces (INF) treaty, all Soviet SS20 missiles will be removed and destroyed within three years, as will the U.S. missiles that eventually were deployed in response.

U.S. officials claim the treaty to wipe out these weapons stems from the determination of North Atlantic Treaty Organization members, who in 1979 set in motion a "dual track" process that authorized deployment of ground-launched cruise missiles and Pershing II missiles in Europe and called for negotiations aimed at getting the Soviets to remove some of their SS20s.

Secretary of State George P. Shultz said Sunday that "it took the resolve of the NATO alliance, the willingness to deploy these {U.S.} weapons and to stand up to {Soviet deployment of the SS20}, that brought about a successful negotiation for what we sought out to achieve in the first place."

At the time of the dual-track decision, it was the deployment of U.S. weapons -- not the prospect of new negotiations -- that most interested the people who made the decision.

The ground-launched cruise missile was seen as an inexpensive alternative to U.S. and allied aircraft in Western Europe. A low-flying, winged rocket, the cruise missile could slip easily through Soviet air defenses to strike vital targets behind enemy lines.

The Pershing II missile was regarded by many officials as a straightforward modernization of the existing Pershing I deployed in West Germany. Few officials anticipated that its ability to reach targets near Moscow in less than eight minutes would create so much anxiety among Soviet military officials and West European peace groups.

Many officials never expected the arms negotiations to succeed. "I can't say I really thought this is how it would turn out," David McGiffert, a former assistant secretary of defense who chaired the NATO committee that crafted the dual-track decision, said yesterday.

McGiffert said the arms negotiations were widely viewed at the time as "a necessary political component" of NATO's decision to deploy the Pershing II and the ground-launched cruise missile, as opposed to a serious diplomatic endeavor with a strong chance of success.

Spurgeon Keeny, who was then deputy director of the U.S. Arms Control and Disarmament Agency, said he thought the negotiations were little more than "a political necessity" intended to make the bitter medicine of nuclear weapons more palatable to a skeptical European audience.

The INF treaty goes well beyond the modest arms limitations envisioned in the dual-track decision. Instead of the "more stable overall nuclear balance at lower levels" of weapons sought in 1979, the INF treaty will permanently eliminate whole categories of land-based medium-range and shorter-range missiles.

This surprising outcome came in large part from the Reagan administration's decision in 1981 to alter the course of the negotiations by endorsing the "zero option" demanding the elimination of all medium-range missiles on both sides.

Wide skepticism greeted the administration's decision to embrace a position first urged by West Germany's liberal Social Democratic Party and by European peace groups.

Much of the skepticism was generated by word that the chief advocate of the "zero option" within the administration was Assistant Defense Secretary Richard N. Perle, widely regarded as an opponent of arms treaties.

Former secretary of state Alexander M. Haig Jr. said he predicted at the time that the proposal would "generate the suspicion that the United States was only interested in a frivolous propaganda exercise or worse, that it was disingenuously engaging in arms negotiations simply as a cover for a desire to build up its nuclear arsenal."

Two years later, the Reagan administration backed away from the demand for elimination of INF weapons by proposing "interim" reductions to "the lowest possible number."

This proposal, which was more in keeping with the original dual-track decision than the zero option, attracted praise in Europe but ensnared U.S. and Soviet negotiators in a protracted dispute about how many weapons could remain on each side.

Gorbachev finally broke the negotiating deadlock by deciding last February to endorse a "zero option" for INF weapons in Europe and to proceed with an INF agreement despite continuing disagreement over strategic weapons and the Reagan administration's Strategic Defense Initiative.

The decision, which came two days after Moscow broke its 19-month unilateral moratorium on nuclear tests, apparently resulted from a compromise between Gorbachev and Soviet military officials.

Gorbachev made another key concession in July, when he agreed to a U.S. demand that missiles capable of reaching targets in Asia be included in the pact. He also agreed to a demand initially made by the West Germans that shorter-range missiles be included.

As more barriers to an agreement dropped, surprised U.S. officials concluded that the Soviets were far more anxious to negotiate the elimination of the Pershing II than anyone had anticipated in 1979.

Many also believed that Gorbachev saw the political value of concluding an arms treaty with a conservative U.S. president, even if it was largely on U.S. terms.Washington Post staff writer Gary Lee contributed to this report.