A report yesterday on the Strategic Defense Initiative omitted part of a statement by John D. Steinbruner, director of foreign policy studies for the Brookings Institution. Asked why the Soviet Union opposes the "Star Wars" plan so strongly and is so eager to negotiate on the subject, he replied, "The Soviets understand the prospects of an effective defense are very small." They are concerned, he added, because "SDI involves a surge of technology across the board . . . . Most of it will show up in the offense."

On March 23, l983, President Reagan surprised the world -- and all but a few of his closest advisers -- by announcing a high-priority research and development program to find ways of intercepting and destroying enemy nuclear missiles before they could hit the United States.

The Strategic Defense Initiative (SDI) quickly captured the public imagination as the "Star Wars" plan because of its reliance on exotic weapons in outer space. Reagan called it "a vision of the future which offers hope" to break out of the grim postwar balance of terror, in which the two nuclear superpowers have achieved a tenuous security by threatening mortal retaliation on each other in case of attack.

The leader of the Soviet Union at the time, Yuri V. Andropov, responded with unusual speed and bluntness from the Kremlin. Just four days later, Andropov personally denounced Reagan's plan as threatening to "open the floodgates to a runaway race of all types of strategic arms, both offensive and defensive."

Just the U.S. intention to develop means of stopping Soviet retaliatory strike, Andropov charged, "is a bid to disarm the Soviet Union in the face of a U.S. nuclear threat."

Beginning with that exchange of hopes and fears 21 months ago, the world entered a new age of uncertainty in the strategic dialogue about the most powerful weapons of the two most powerful nations.

Rarely has there been such a dramatic example of words and concepts outpacing tangible facts and capabilities. Reagan's proposal, it has now been learned, was adopted in a highly personal, secret and almost accidental manner. It has long been clear that his announcement preceded his administration's own studies to determine the technical possibilities, practical objectives and strategic rationale of the plan he already had announced. In his initial statement Reagan said the effort probably would take decades of work and "may not be accomplished before the end of this century."

In terms of tangible scientific and military development, very little has changed since March 1983. A White House briefer told reporters yesterday that as of now, "SDI does not" exist.

Nevertheless, the mere possibility that potent American technology might eventually be able to fulfill Reagan's "vision" has cast a cloud of doubt over the fundamental premises and equations that underlie nearly two decades of U.S.-Soviet arms control negotiations and treaties. In the minds of policy-makers and negotiators, a new unknown has been added to the deadly calculus of nuclear threat and counterthreat.

Reagan's "Star Wars" plan will be at the heart of the discussions next week in Geneva between Secretary of State George P. Shultz and Soviet Foreign Minister Andrei A. Gromyko. Even after pulling out of negotiations on offensive nuclear weapons near the end of 1983 and declaring it impossible to do business with Washington, the Soviets persistently called for negotiations about outer space weaponry.

Senior U.S. officials said it is clear from the still-secret text of the Nov. 17 letter from Soviet leader Konstantin U. Chernenko, which proposed the Geneva talks, that the highest Soviet priority there will be on stopping the U.S. drive into space. This fact provides the main potential leverage of the United States in renewed bargaining with the Soviet Union.

How and why did the issue of strategic defense return to the U.S.-Soviet bargaining table after an absence of 12 years? What brought Reagan to seize an idea that had been rejected as impractical by most experts and that his secretary of defense, Caspar W. Weinberger, scorned just four months before it was announced at the White House? What do the arguments and developments portend for arms control, the arms race and relations between Washington and Moscow?

In considering the relationship between offensive and defensive weapons in the strategic arena, it is vital to understand that the incredible power of nuclear weapons accords tremendous advantage to the attacker. (The largest nuclear weapons pound for pound pack about 1 million times the power of conventional explosives, hence the term "megatons" to measure their force.) The predominant view until the mid-1960s was that no defense was possible against this destructive force, especially when deployed on long-range missiles that travel in space.

By 1966, however, radars and other defensive technology had advanced to the point that the Soviets were working on a limited antimissile defense around Moscow that would use missiles to hit other missiles. In the United States, President Lyndon Johnson was under growing pressure to spend billions on an elaborate U.S. defense system, despite the clear prospect that both Soviet and American defensive programs would be overwhelmed by rapid developments on the offensive side.

Johnson, to head off a costly and seemingly futile competition, proposed to the Soviets in December 1966 talks on limiting antiballistic missile (ABM) systems on both sides. Moscow, which had a long tradition of reliance on defense, replied that it was necessary to deal with both offensive and defensive arms in considering limitations because of the integral relationship between them.

The SALT I negotiations that began in November 1969 finally produced a treaty in mid-1972 sharply limiting the ABM programs on each side and a separate "interim agreement" that was to be the first step toward serious limitation of offensive weapons. By the end of the negotiations, in a reversal of positions, Washington was pushing for offensive limitations to go along with the defensive restraints.

By agreeing in 1972 to all but abandon strategic defense, the two powers in effect enshrined their mutual vulnerability and the threat of retaliation as the basis for their security. Being held hostage by an adversary's power, while holding him hostage as well, has been a frustrating, unsettling and in large measure unnatural situation, especially amid sharply intensified political hostility between the two powerful nations.

In a news conference shortly after his March 1983 announcement, Reagan compared the prevailing situation to "people facing themselves across a table, each with a cocked gun, and no one knowing whether someone might tighten their finger on the trigger." He said it was inconceivable to him that the great nations of the world would continue this indefinitely.

In a pamphlet made public at the White House yesterday, Reagan cited two reasons for departing from the longstanding acceptance of vulnerability. For one thing, he charged, the pace of a continuing Soviet offensive and defensive military buildup "has upset the balance in the areas of greatest importance during crises." For another thing, he said, "new technologies are now at hand which may make possible a truly effective non-nuclear defense."

Reagan, like many other conservatives, has long been acutely uncomfortable with "mutual assured destruction," which acquired the sarcastic acronym, MAD. In December 1980, during the transition between the election and his inauguration as president, Reagan raised the possibility of an antimissile defense through space-based lasers with former astronaut Harrison Schmitt, then a Republican senator from New Mexico.

"He asked me what were the technological possibilities of altering that strategic policy toward one of protection rather than mutually assured destruction," recalled Schmitt, who told Reagan that much could be done if the effort became a matter of national policy.

A variety of officials and former officials interviewed recently said they believe that Reagan's interest has been reinforced by Dr. Edward Teller, the father of the U.S. hydrogen bomb, who long opposed the ABM treaty and sponsored the development at a government laboratory in northern California of X-ray lasers generated by nuclear weapons. The two men were acquainted during Reagan's days as governor, and Teller visited Reagan at the White House in September 1982 to discuss strategic defense.

Another influence, by most accounts, was the High Frontier proposal for a non-nuclear space based defense developed by a group of conservatives under retired Lt. Gen. Daniel O. Graham, who was a defense policy adviser to Reagan during the 1980 campaign. Graham said in an interview that in March 1981 he spoke briefly to Reagan about a strategic defense in space and was told to take his ideas to Weinberger.

In mid-January 1982 Reagan was briefed on a lengthy High Frontier report, privately financed from $500,000 in contributions, which advocated "a layered strategic defense" to replace mutual assured destruction. Graham, who was not at the briefing, later reported general receptivity from Reagan, "only weak support" from White House science adviser George A. Keyworth II and "actual hostility" from the Defense Department.

A Pentagon study under the supervision of Dr. Richard D. DeLauer, undersecretary of defense for research and engineering, was highly skeptical of the High Frontier proposal for strategic defense in briefings for Weinberger and the plan's advocates. In response to a protest from Graham, Weinberger wrote back to the retired general on Nov. 24, 1982:

"I see no disparity between wanting a new policy and not having the means to achieve it. [DeLauer] and I agree with you that a policy based on effective space defense would enhance national security. However, we differ from you on the availability of technology to support such a policy. Although we appreciate your optimism that 'technicians will find the way and quickly,' we are unwilling to commit this nation to a course which calls for growing into a capability that does not currently exist."

A new ingredient that turned out to be crucial involved the Joint Chiefs of Staff, who were increasingly concerned late in 1982 and early in 1983 over the troubles of the nation's latest proposed addition to land-based strategic offensive forces, the MX. The "dense pack" basing mode for the MX proposed by Reagan and the Pentagon was rejected on Capitol Hill. Nuclear freeze advocates were marching and a substantial majority in the House was voting against the entire program.

It seemed to the nation's uniformed military leaders, as they surveyed the situation in private discussions, that continued reliance only on strategic offensive forces was a questionable policy for the future. "The chiefs considered there was a lack of balance," said an informed military official who asked not to be identified.

Weinberger was informed of the chiefs' view that a more intensive program of investigating a strategic defense should be considered, the official said. But he and others said there was no formal recommendation at this point for a full-scale national effort.

Reagan met the Joint Chiefs in the presence of Weinberger and Robert C. McFarlane, then deputy national security affairs adviser, on Feb. 11, 1983, the day of Washington's biggest snowstorm in four years. The main subject of the meeting was to be a new basing mode for the MX. But at the mention of strategic defense as an option to be considered, according to one participant, Reagan displayed an immediate and strong interest.

According to a second-hand account, Reagan kept the discussion on strategic defense for close to half an hour, though it had been expected to be much briefer.

One participant told a friend later that, as the discussion proceeded, Reagan asked those around the table, "Would it not be better to defend lives than to avenge them?" To this observer, familiar with the president's ways, the ring of that rhetoric signified a policy change whose time had come.

Reagan was due to make a long-postponed address to the nation late in March to defend U.S. military budgets and programs, especially the MX, against growing attack. In advance of the speech, White House aides were searching for a new element -- they called it "MX Plus" -- to make the case more palatable to the public.

This "coda," as some called it, or "speech insert," as others knew it, was prepared in great secrecy by a very small group of aides. Few people outside this group were consulted. One who was informed, and thought the future SDI to be a bad idea, recalled that one of Reagan's closest advisers -- now a leading public advocate of the program -- said he was "thrown out of the Oval Office" when he tried to persuade Reagan to desist.

Weinberger, who was traveling abroad, reportedly argued until the last minute against the SDI announcement, ostensibly on grounds that it would detract from the intended effect of the overall speech. White House aides suspected that Weinberger was actually less than enthusiastic about the program, though he is now one of its most vocal advocates.

DeLauer, the Pentagon's research chief who was most familiar with and responsible for military-sponsored work on the technology being addressed, recalled that he did not see the Reagan announcement until the day before it was broadcast nationwide. "We had no major input," he said.

DeLauer recalled that a hasty study had been put together by some members of the President's Science Advisory Council under the supervision of science adviser Keyworth.

"It was a concept" more than anything else, said DeLauer, noting that "essentially the same programs" were pursued before and after the announcement. Before March 23, the Pentagon was planning to spend $1.2 billion for defensive technology in fiscal year 1985. After the high-level studies that followed the address, the spending was upped to $1.6 billion with "a little more to the Air Force, a little less to the Army," he said.

Though military programs were changed little, the change in philosophy and objectives announced by Reagan generated strong reactions among scientists, politicians and arms control advocates in the United States. But the strongest reactions were from the Soviet Union.

Only a day or two after the address, Soviet negotiator Viktor P. Karpov raised the issue with U.S. negotiator Gen. Edward Rowny in the Geneva strategic arms reduction talks (START), according to administration sources. The Soviet negotiator charged that Reagan's program would violate the 1972 ABM treaty, which Rowny denied. Nevertheless, insisted Karpov, "the spirit" of the treaty at least was being undermined.

The U.S. negotiators insisted, as Shultz is expected to repeat to Gromyko next week, that SDI is an attempt to gain greater strategic stability rather than to undermine the tenuous existing patterns. The Soviets strongly disagree.

One of the problems in negotiating about "Star Wars" is that, because of its history and vague nature, its actual dimensions are still unknown beyond a five-year research program estimated to cost $26 billion.

Former defense secretary Harold Brown said it could turn out to be either: a full-scale defense of the U.S. population against nuclear attack, which could cost $1 trillion and still be "an impossible dream"; a thin defense of U.S. cities against small nuclear attacks or an accidental launch, which would cost $100 billion; or a limited defense of missile sites costing $10 billion or more.

If an effective "Star Wars" plan is as impractical as portrayed by some U.S. private experts, why are the Soviets so strongly opposed and so eager to negotiate on the subject? In answer to that question, John D. Steinbruner, Brookings Institution director of foreign policy studies, told reporters that "the Soviets understand the prospects of an effective defense are very small."

Despite the strong Soviet interest in curbing SDI, all indications are that Shultz at Geneva will be under instructions to reject any negotiated arrangement that would crimp SDI in substantial fashion.

Reagan, in his pamphlet released yesterday, described his "Star Wars" plan as "both militarily and morally necessary." He called it an effort to strengthen world peace "by rendering ballistic missiles impotent and obsolete."

Other officials are less definite. The Pentagon last month awarded about $10 million in contracts for "system architecture studies" to determine what could be done and at what cost. A White House official yesterday described the plan as it exists as only research and said transforming "the vision" into reality would be for Reagan's successors to decide.