For months, Manning's defence team has demanded access to reports by government agencies, including the CIA, that assessed the effect of the leak of classified documents to the WikiLeaks website.
Manning is accused of passing on a massive trove of files to WikiLeaks but his lawyers believe the reports will show the alleged disclosures had no major effect on the country's national security.
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Judge Denise Lind ruled that government prosecutors must provide "damage assessment" reports from the CIA, the State Department, the FBI, the Office of the National Counterintelligence Executive (Oncix) and other documents that were relevant for the defense.
The judge, agreeing with a request from Manning's lawyers, also ordered the prosecution to give a detailed account showing it had met legal obligations to share all pertinent evidence with the defence.
In a statement of "due diligence," the prosecutors would have to show what documents they had obtained and why any files were not shared with the defence.
The judge imposed a deadline of July 25 on the prosecutors but indicated she would be willing to give the prosecution more time to produce the statement. Defence lawyers would also have a chance to request more time to review any new evidence that was passed on.
Manning's civilian lawyer, David Coombs, had argued earlier Monday that government prosecutors had displayed a "pattern" of obstruction on document requests that "should cause alarm to the court."
Activists for Manning who attended the hearing at Fort Meade, northeast of Washington, welcomed the decision.
"It's definitely heartening that David Coombs' arguments are resonating with the judge," said Zach Pesavento, a spokesman for the Bradley Manning support network.
"It appears they (prosecutors) may have been essentially lying to the court," about the availability of some documents requested by the defence, he said.
"We believe that truth is ultimately on Bradley's side," he added.
Manning, 24, could spend the rest of his life in prison if convicted of aiding the enemy by handing hundreds of thousands of classified documents -- including military logs from Iraq and Afghanistan and sensitive diplomatic cables -- to the WikiLeaks website. He has not yet entered a plea.
