WASHINGTON - President Obama yesterday ordered the federal government to rethink how it protects the nation's secrets, in a move that was expected to declassify more than 400 million pages of Cold War-era documents and curb the number of records hidden from the public.

Among the changes is a requirement that every record be released eventually and that federal agencies review how and why they mark documents classified or deny the release of historical records. A National Declassification Center at the National Archives will be established to assist them and help clear a backlog of the Cold War records by Dec. 31, 2013.

Obama also reversed a decision by President George W. Bush that had allowed the intelligence community to block release of a specific document, even if an interagency panel decided the information wouldn't harm national security.

Advocates for a more open government were cautiously cheering the move.

Obama said he expected the order would produce "measurable progress" toward greater openness in government while also protecting the nation's most important secrets.

"I will closely monitor the results," he promised.