WASHINGTON — If it had been accepted, President Reagan's 1986 offer to the Soviet Union to ban all strategic nuclear missiles would have left the United States vulnerable to Soviet attack in the absence of subsequent large defense budget increases and a major buildup of strategic defenses, according to a report by the Joint Chiefs of Staff made public Friday.
The report, disclosed by Sen. Sam Nunn (D-Ga.), tended to support criticism in many quarters at the time that Reagan had acted precipitously in making the sweeping proposal to Soviet leader Mikhail S. Gorbachev at their October, 1986, summit meeting in Reykjavik, Iceland.
The offer, made without first being circulated among key government agencies, faded into obscurity when Gorbachev demanded as a condition of his acceptance that the United States pledge to severely restrict research on the Strategic Defense Initiative, Reagan's plan for space-based missile defenses.
The Pentagon study demonstrated that the proposal "could have posed serious national security problems and compelled large budgetary increases," said Nunn, the influential chairman of the Senate Armed Services Committee and a critic of Reagan's Reykjavik proposal.
"The lesson we should draw . . . is that the President should obtain the . . . advice of his top military advisers before, and not after, making such far-reaching proposals to the Soviet Union," Nunn said.
Release of the report, which reflects the conclusions of Adm. William J. Crowe Jr., the nation's top-ranking military officer as chairman of the joint chiefs, comes as the Senate is conducting ratification hearings on the more modest arms reduction agreement signed at last month's Reagan-Gorbachev meeting in Washington. That treaty would eliminate ground-launched medium-range missiles from the superpowers' arsenals.
The large budget increases that Reagan's proposal would have made necessary pose a sharp contrast to the current Pentagon effort to cut $33 billion from its 1989 budget request to help reduce the huge federal budget deficit.
Congress required the joint chiefs to provide the assessment in an amendment to the defense bill for fiscal 1988.
Under the plan Reagan proposed in Iceland, the United States would have agreed not to withdraw from the 1972 Anti-Ballistic Missile Treaty for 10 years, and both nations would have eliminated all their land- and submarine-based missiles during the same period.
In the wake of the Iceland summit, many Administration critics criticized the proposal as a public relations gesture rather than a serious negotiating position.
Nunn argued that the proposal would have left the United States and its allies vulnerable to the Soviets' superior non-nuclear forces.
"What would (the United States) do, under these conditions, to correct the conventional balance, which now favors the Soviet Union so dramatically?" Nunn asked.
The joint chiefs' report said that the proposal, had it been accepted, would have set off a new arms race in which both nations would have invested heavily in bombers and submarines equipped to carry cruise missiles, neither of which would have been covered by the accord.
At the same time, the United States, which "could not afford to remain relatively defenseless against nuclear attack by bombers and cruise missiles," would have had to erect costly defenses along the nation's borders to counter those weapons, the study said.
Also, the joint chiefs maintained, "Given the potential for Soviet cheating, even in a U.S.-Soviet zero-ballistic missile world, U.S. ballistic missile defense forces would be required to protect against any covertly held Soviet ballistic missiles."
The version of the report made public did not provide a specific projection of costs for the defense improvements, but it said the expense would "exceed any realistic projection of near-term defense budgets."