WASHINGTON (AP) - More than 400 million pages of Cold War-era documents could be declassified as the federal government responds to President Barack Obama's order to rethink the way it protects the nation's secrets.

Among the changes announced Tuesday by Obama 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 the release of a specific document, even if an interagency panel decided the information wouldn't harm national security.

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

"Everything will depend on implementation," said Steven Aftergood, director of the Project on Government Secrecy at the Federation of American Scientists. But the order "has tremendous potential to reduce the level of secrecy throughout the government."

In a memo to agency heads, Obama said he expects that the order will 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.

The still-classified Cold War records would provide a wealth of data on U.S.-Soviet relations, including the Soviet invasion of Afghanistan, the fall of the Berlin Wall, diplomacy and espionage. A Soviet spy ring in the Navy led by John Walker headlined 1985, which became known as "The Year of the Spy."

On his first day in office, Obama instructed federal agencies to be more responsive to requests for records under the Freedom of Information Act and he overturned an order by Bush that would have enabled former presidents and vice presidents to block release of sensitive records of their time in the White House.

The government spent more than $8.21 billion last year to create and safeguard classified information, and $43 million to declassify it, according to the Information Security Oversight Office, which oversees the government's security classification. The figures don't include data from the principal intelligence agencies, which is classified.