President Dwight D. Eisenhower gave authority in 1957 to senior U.S. military commanders to retaliate with nuclear weapons if the president could not be reached or was otherwise unable to respond to a nuclear attack against the United States, according to declassified documents released this week.

The authority to launch nuclear weapons was only to be used "when the urgency of time and circumstances clearly does not permit a specific decision by the president, or other person empowered to act in his stead," according to a 1959 memo outlining the policy for what is known as "pre-delegation authority." The memo, sent to the head of the Strategic Air Command from the Joint Chiefs of Staff, was among 18 formerly classified documents obtained by the National Security Archive, a public interest documentation center.

Defense specialists have long assumed the existence of pre-delegation authority to use nuclear weapons, but the released government memo and internal working papers mark the first time the issue has been publicly documented. Pre-delegation for a nuclear attack has been among the most highly classified and controversial issues in government since atomic bombs entered the U.S. military arsenal in 1945, and remains so today.

A senior National Security Council official in the Clinton administration said yesterday that the White House would not comment on the pre-delegation issue. Other sources said some elements of pre-delegation still exist today, and three recently retired officials who specialized in nuclear weapons policy refused to comment on the matter. "It is still sensitive," one said yesterday.

The documents released this week suggest that Eisenhower's pre-delegation instructions were continued by Presidents John F. Kennedy and Lyndon B. Johnson. In a top secret memo in March 1964 to Secretary of Defense Robert S. McNamara, Johnson said he authorized putting into effect "the updated Instructions for Expenditure of Nuclear Weapons in Emergency Conditions," which appears to be a revision of the Eisenhower plan.

William Burr of the National Security Archive said the declassified materials were obtained through Freedom of Information Act requests filed as long as five years ago. The documents were housed in various presidential libraries. He said his organization is "trying to document the history of U.S. nuclear weapons programs, which has always been very shadowy and unclear."

"Nuclear issues are key to the history of the Cold War," Burr said, "and a lot of issues are still not understood."

It is unclear from the documents, which were heavily redacted before their release, how widely among military commanders authority was granted to launch nuclear weapons in an emergency.

The documents released this week shed light on another facet of U.S. nuclear policy during the Cold War: the conditions under which nuclear weapons could be launched from allied counties. Under the Eisenhower instructions, U.S. nuclear weapons could be used from bases abroad only in line with "agreements or understandings, if any" that existed with the governments of those countries.

Subsequent documents describe agreements varying widely from country to country. A top secret 1961 memo from a State Department official to Kennedy's national security adviser, McGeorge Bundy, enumerates "existing arrangements" then in effect with Britain, France, West Germany, Spain and some NATO countries.

For example, the memo states that in 1961 the United States required a "joint decision" with the British prime minister "personally" before U.S. forces with nuclear weapons could operate from bases in England. In addition, U.S. policy was to consult with the British "if possible" before using nuclear weapons anywhere in the world. A special effort was to be made to consult with London before employing submarine-launched Polaris long-range nuclear missiles because some of those submarines were to be based in Scotland.

With the French, in 1961, U.S. policy was to consult with Paris "unless an attack was so imminent that survival of U.S. is at stake," the memo said. Consultation with France was to be done using a direct phone "established for this purpose," according to the memo. The use of bases in France for nuclear weapons "is subject to joint decision," the memo said.

With regard to NATO countries such as Denmark, Greece, Italy, the Netherlands, and Turkey, launching nuclear weapons from bases in those countries would be determined in accord with existing NATO plans, which the memo did not specify. There was to be "no limitation" in 1961 on the use of nuclear weapons from U.S. bases in West Germany, Morocco and Spain, the memo said.

The most likely situation envisioned 40 years ago for pre-delegation authority was in the event of a nuclear "first strike" attack by the Soviet Union on the United States in which "immediate communications have become impossible between the president and responsible officials of the Department of Defense," according to the 1959 instructions.

The emergency "instructions regarding retaliation" specify that the head of the Strategic Air Command "must assure such delegation of authority is not assumed through accident or misinformation," that retaliation be "against the enemy identified as responsible for the attack," and that delegation would remain effective "only until it is possible to communicate with the president or a person entitled to act for him." Pre-delegation was also studied in event of an air attack against the United States for use of nuclear-tipped U.S. antiaircraft weapons, which long ago were removed from the Pentagon inventory. The released Eisenhower instruction does not indicate whether there finally was pre-delegation for this weapon.

The 1961 Kennedy administration document said that U.S. air defense interceptor aircraft could overfly Canada when a nuclear readiness alert was in effect. But they "may expend nuclear weapons only after authorization by the president of the U.S. and the prime minister of Canada," the memo said.

Eisenhower wanted additional Pentagon study of whether pre-delegation should take place with attacks against U.S. ships in international waters, according to one memo.

Eisenhower's decisions about whether there should be pre-delegation when American forces stationed abroad were subject to either a nuclear or a conventional attack remain classified.