US army private Bradley Manning faces a possible 20 years in prison after a judge accepted his guilty plea on 10 charges involving illegal possession or distribution of classified material.
Military judge Colonel Denise Lind accepted his plea on Tuesday to 10 of 22 charges against him.
Pte Manning was allowed to plead guilty under military regulations instead of espionage law, which knocked the potential sentence down from 92 years.
He will not be sentenced until his court-martial on the other charges is concluded.
But military prosecutors said they plan to move forward with a court-martial on 12 remaining charges, including aiding the enemy, which carries a potential life sentence.
Pte Manning directly admitted leaking the material to WikiLeaks for the first time and detailed the frustrations that led him to do it, saying he wanted to expose the human cost of US wars in Afghanistan and Iraq.
"I began to become depressed at the situation we found ourselves mired in year after year. In attempting counterinsurgency operations, we became obsessed with capturing and killing human targets on lists."
He said he was disturbed by the conduct of the Afghanistan and Iraq wars and the way US troops treated its people.
"I believed that if the general public, especially the American public, had access to the information … this could spark a domestic debate on the role of the military and our foreign policy in general," said Pte Manning.
He said he was appalled by a 2007 combat video of an assault by a US helicopter that killed 11 men, including a news photographer.
The Pentagon concluded at the time that troops mistook the camera equipment for weapons.
"The most alarming aspect of the video to me was the seemingly delightful bloodlust the aerial weapons team happened to have," he said, adding that the soldiers' actions "seemed similar to a child torturing ants with a magnifying glass."