Some secrets, it turns out, are too old or too big to keep - even for the Bush administration, which has made a crusade of rooting out leaks and clamping down on information on the inner workings of government.
In the new year, the CIA, FBI, state department and more than 80 other government agencies that handle state secrets will declassify hundreds of millions of pages of documents under a new policy that institutes an automatic release of material after 25 years.

Within those documents lie the most turbulent episodes of the 20th century: the 1979 Soviet invasion of Afghanistan and the Vietnam war, the CIA"s unauthorised experiments with LSD and its internal thinking on a raft of investigations into coups and assassinations overseas, and the FBI"s hunt for communist sympathisers on US soil.

The release, awaited by scholars and journalists, goes against the grain for the president, George Bush, and the vice-president, Dick Cheney, who has argued that the disclosure of information from the White House erodes presidential power.

The decision to release documents after 25 years was made in 1995 under President Bill Clinton, although the Bush administration managed to delay it. "I was pleasantly surprised," said Steven Aftergood, who runs a project on government secrecy for the Federation of American Scientists. "I could have easily imagined this administration saying: "Oh, no we can"t possibly adopt an automatic declassification policy. That will only assist the terrorists"."

Until now, material could remain secret indefinitely unless researchers lodged a specific request under freedom of information regulations. But declassification does not guarantee documents will be made public. Government agencies can withhold them on privacy grounds, to protect an intelligence source, or to avoid compromising an ongoing investigation.

The FBI has been notoriously stringent about exercising that prerogative, refusing to release documents on the assassination in Washington of the Chilean diplomat Orlando Letelier by agents of the Pinochet regime on the grounds that investigators were still pursuing leads.

However, advocates of greater government accountability say an automatic release of documents remains an important step forward.