Historians have long believed that American presidents going back to Eisenhower gave some commanders authority to launch a nuclear counter-attack without further approval. On Friday, newly declassified documents spelling out that policy were made public.

The National Security Archive, a non-governmental library that collects declassified documents and publishes them, posted on its Internet page government papers going back to the 1950s shedding light on when a commander could launch an attack on his own.Bruce Blair, senior fellow at the Brookings Institution and author of several books on nuclear strategy, said from Dwight Eisenhower's day on the policy stipulated that a commander had first to confirm with high confidence that a nuclear attack had occurred on American soil and had to be unable to reach the president, the vice president or others in the line to succeed them.

"Under those circumstances, he could decide to order the launch and also could determine the scale of attack and the targets," Blair said. "He becomes in effect the stand-in president."

Left unanswered is one key question: Has the "predelegation" policy been revoked and, if so, by which president? Or does it remain in effect?

William Burr, a senior analyst at the National Security Archives, said the documents establish "that during the most dangerous phases of the U.S.-Soviet confrontation top military commanders had presidentially authorized instructions providing advance authority to use nuclear weapons under specified emergency conditions."

Burr obtained the documents through the Freedom of Information Act, a process he started in the early 1990s. The pages released are heavily censored and many others remain classified.

The only document among those released that bears a president's signature is one signed by Lyndon B. Johnson, dated March 26, 1964, labeled "top secret" and addressed to his secretary of defense, Robert S. McNamara.

"I accept the recommendation . . . and hereby authorize you to put into effect the updated `Instructions for Expenditure of Nuclear Weapons in Emergency Conditions' which were brought to me by the Joint Chiefs on March 4, 1964," it said. "It is my understanding that the redrafted instructions are basically the same as those approved by President Eisenhower and continued in effect by President Kennedy."

Students of the subject are uncertain whether the policy remains in effect.

"Having spent a lot of my professional career investigating issues of nuclear control - and having interviewed people who had been given delegated authority (to issue an order for nuclear retaliation) - I believe that the Eisenhower precedent, with some variations on the theme, continued, probably to the present time, although the scholarly trail for me ended with the Reagan administration," Blair said.