The Air Force’s “Reports on Soviet Air Power and Strategic Nuclear Weapons,” about 2,500 pages in all, were produced between 1952 and 1955 — but not until earlier this year were 2,210 pages made public. The release is part of a massive effort at the National Archives and Records Administration to clear a backlog of nearly 400 million pages of material that should have been declassified a long time ago.

“All of these pages had been piling up here, literally,” said Sheryl J. Shenberger, a former CIA official who is the head of the National Declassification Center (NDC) at the National Archives. “We had to develop a Costco attitude: We had 400 million pages . . . and we have three years to do them in.”

The impetus to clear the backlog, which included material dating back to World War II and before, was a 2009 executive order issued by President Obama that created the center. At the time, David S. Ferriero, the archivist of the United States, said the federal government had reached “a watershed moment” in records declassification.

“The current backlog is so huge that Americans are being denied the ability to hold government officials accountable for their actions,” Ferriero said. “By streamlining the declassification process, the NDC will usher in a new day in the world of access.”

All of the backlogged documents date back 25 years or more, and most are Cold War-era files from the departments of Defense, State and Justice, among other agencies. The CIA manages the declassification of its own files.

The nearly 400 million pages of material had already been reviewed by the department or agency that produced them. But they could not be released until every other agency whose information might be contained in a document had also looked at them and signed off on a declassification decision. That process proved to be a bureaucratic stovepipe through which nothing much moved.

To cut through the mountain of paper, the NDC has introduced a risk-management approach to the documents. Instead of attempting to look at every document, all the relevant agencies agreed to look at a small sample of a particular series. If the reviewers, drawn from all agencies, decide by looking at the sample that the earlier agency review was a good one, then the remainder of the documents that the agency had cleared is made public.

In a memo accompanying the executive order, the White House also stated that original declassification decisions could only be referred for further review if the material is found to contain information relating to the design of weapons of mass destruction or the identification of human intelligence sources.

If such material is found, another sampling occurs. And if more information about WMD or intelligence sources is discovered in a second sample, then the entire collection gets sent back for closer review.

Shenberger estimates that about 80 percent of the documents will be cleared for release after the first sampling, and 20 percent will need further review. Of that 20 percent, she said, half will likely be released.

About 217 million pages are in various stages of assessment by more than 100 personnel from the National Archives and various agencies. The reviewers, based on the samplings, have so far judged that 108 million pages were assessed correctly by the originating agency. Already, more than 20 million pages have been released to the public, including the Pentagon Papers and one of the oldest classified materials in the United States, a World War I-era document on invisible ink. Most files are not as momentous or intriguing, but they are of significant value to historians.

The recently declassified Air Force reports on Soviet air power, for instance, capture some of the uncertainty about the Kremlin’s intentions. In a 1955 presentation at the Air War College, Col. William A. Adams told his audience that “[w]e have quite a bit of confidence in our estimate of the magnitude of the Soviet threat to the U.S. . . . But as far when, and how, and even if, the Soviets will employ their capability to attack the U.S., it is very hard to estimate.”

The declassification effort is earning some plaudits from usually skeptical observers.

“They are bringing a new degree of order and efficiency to what has been a disorderly process,” said Steven Aftergood, who directs the Project on Government Secrecy at the Federation of American Scientists. “The various agencies understand the imperative from above and they are motivated.”

Aftergood said, however, that the White House initiative only affects documents that go back 25 years or more and “does not deal with contemporary records, which are also in need of declassification review.”