In her special findings explaining the reasoning behind Manning's convictions on major counts, the judge, Col. Denise Lind, said the leaks were of "a heedless nature that made it actually and imminently dangerous to others," Courthouse News reported.
But the judge did not explain why she found Manning not guilty of "aiding the enemy."
Manning faces a possible 90 years in prison for giving massive amounts of classified information to the WikiLeaks website.
Lind is expected to begin hearings on Manning's punishment Monday.
Manning apologized Wednesday for the "unintended consequences" of his massive leak of classified information.
Manning told Lind at Fort Meade, Md., he is sorry he "hurt people and hurt the United States," the Los Angeles Times reported.
Lind convicted Manning of violations of the Espionage Act for stealing and releasing the documents.
Manning was arrested in 2010 in Iraq, where he had access to the confidential information.