The US Army private accused of providing secret documents to the WikiLeaks website pleaded guilty tonight to misusing classified material he felt "should become public".
However he denied the top charge of aiding the enemy.
Private First Class Bradley Manning, 25, entered the pleas prior to his court martial, which is set to begin on 3 June, in a case that centres on the biggest leak of government secrets in US history.
Military judge Colonel Denise Lind accepted the guilty pleas late in the afternoon.
Manning pleaded guilty to a series of ten lesser charges that he misused classified information and faces a maximum of 20 years in prison for those offences.
"I believe that if the general public... had access to the information... this could spark a domestic debate as to the role of the military and foreign policy in general," Manning, dressed in full military uniform, testified calmly.
Reading from a 35-page statement as he remained seated next to his lawyers, he described his feelings after he submitted the secret information to WikiLeaks.
"I felt I accomplished something that would allow me to have a clear conscience," said Manning, who spoke under oath for more than an hour.
"This was the type of information... should become public," he said.
At the hearing, through his attorney Manning pleaded not guilty to the most serious charge, of aiding the enemy.
Manning, who has been jailed at Quantico Marine Base in Virginia for more than 1,000 days, could face life imprisonment if convicted of that charge at his June trial.
Under a ruling last month by Col Lind, Manning would have any sentence reduced by 112 days to compensate for the markedly harsh treatment he received during his confinement.
While at Quantico, Manning was placed in solitary confinement for up to 23 hours a day with guards checking on him every few minutes.
Manning admitted to unauthorised possession and wilful communication of classified information from the Combined Information Data Network Exchange Iraq and the Combined Information Data Network Exchange Afghanistan, two military databases.
He called the two tables of documents he sent to WikiLeaks "two of [the] most significant documents of our time".
He also admitted to misuse of documents from the US Southern Command pertaining to Guantanamo Bay, a memo from the United States Army Intelligence Center, and records from a military operation in Farah province in Afghanistan.
One of the classified US military videos he said he leaked showed the 2007 attack by Apache helicopters that killed a dozen people in Baghdad, including two Reuters news staff, photographer Namir Noor-Eldeen, 22, and his assistant and driver Saeed Chmagh, 40.
Manning, an Army intelligence officer, testified that he first tried to give the information to his "local paper," the Washington Post, but when a journalist there was not interested he left a message at The New York Times, which never returned his call.
He then planned to visit the offices of Politico, but when a winter storm cancelled his plans, he turned to WikiLeaks.
Manning was arrested in May 2010 while serving in Iraq and charged with downloading thousands of intelligence documents, diplomatic cables and combat videos and forwarding them to WikiLeaks.
WikiLeaks began exposing the US government secrets in the same year, stunning diplomats around the world and outraging US officials who said damage to national security from the leaks endangered US lives.
WikiLeaks founder Julian Assange has taken refuge in the Ecuadorean Embassy in London since June to avoid extradition to Sweden for alleged sex crimes.