One of the most powerful formal checks on government secrecy is the Freedom of Information Act, which compels the government to produce documents upon demand, with some exceptions. Since 1966, as you might imagine, the scope of the exceptions has been the subject of a significant amount of litigation. As you might also imagine, the government likes to define the exceptions broadly, and public advocacy groups like to define it narrowly.
In general, courts have been reluctant to side with the public in cases involving agencies involved in the national security state, specifically if a "national interest" type of exemption is cited. After all, the executive branch controls what constitutes national security information and decides how to protect it.
A group dedicated to monitoring the Ft. Benning, Ga.-based "Western Hemisphere Institute for Security Cooperation," a military school that trains foreign soldiers, alleged that the Department of Defense illegally withheld information by improperly and overbroadly citing exemptions. The institute used to be known as the School of the Americas; some of its graduates have been implicated in brutalities, and the government has promised to keep those folks with bad human rights records from attending. But the group, SOA Watch, found, based on its previous FOIA requests, that soldiers with questionable human rights records had in fact been allowed to attend the school, and forwarded this information to Congress in 2004. The DoD stopped providing the names to the watchdog group.
It filed a FOIA request in 2011, and the Defense Department originally cited "exemption b(3)," which protects the privacy of people with a statutory right to it, as the reason for not releasing any of the names of students and military units. But after SOA Watch challenged the denial, the Defense Department added "exemption b(6)" to their list of reasons for not disclosing the information. That's the exemption that allows federal agencies to deny respondents information when the secretary of a particular department determines it's in the national interest.
Today, a judge reversed a long-standing habit of deferring on this exemption, and concluded that the FOIA agency has the burden to demonstrate why release would not be in the national interest. The Pentagon did a lot of asserting in its legal briefs, but not a lot of proving. (One claim: If the names of those who attend are released, they might be subject to harm back home, since many of them come from countries where being a law enforcement officer is dangerous. But the DoD did not find a way to prove their names were kept secret in their country of origin.)
There will be an appeal, of course, but the burden is now on the government to say why it must not be burdened with the justification of citing and asserting various FOIA exemptions.
This decision is the second major FOIA win for activists this month. In early April, an appeals court ruled that FOIA agencies must not only acknowledge having received a FOIA request within 20 days of getting one, but they must also give the requester information about what documents will be forthcoming, and which won't be.