Army Pfc. Bradley Manning, who was charged in the largest leak of classified material in U.S. history says he sent the material to WikiLeaks to enlighten the public about American foreign and military policy. (Patrick Semansky/AP)

The Army private charged in the biggest leak of classified material in U.S. history pleaded guilty to 10 charges Thursday and offered an impassioned defense of his actions, arguing that he sought to spark a national debate about what he described as the nation’s obsession with “killing and capturing people.”

The testimony marked Pfc. Bradley Manning’s first detailed account of his disclosure of a trove of U.S. diplomatic cables and military documents in 2010 to WikiLeaks, an anti-secrecy organization he said he approached after he was unable to entice The Washington Post and the New York Times.

Manning’s detailed account of his work as an intelligence analyst in Iraq and his decision to divulge sensitive documents is certain to elevate his folk-hero status among the band of supporters who hail him as a conscientious whistle-blower rather than a traitor.

“We were risking so much for people who seemed unwilling to cooperate with us,” the 25-year-old soldier said, speaking calmly and quietly. It was spawning “frustration and hatred on both sides,” he said, adding: “I became depressed.”

Manning said that WikiLeaks never pressured him to turn over documents and that he came to see the group led by Australian Julian Assange as the best vehicle to dump the war logs and cables into the public domain.

“I believed that if the general public, especially the American public, had access to the information . . . this could spark a domestic debate over the role of the military and our foreign policy in general,” Manning said in a statement he read while seated next to his attorneys in a packed courtroom at Fort Meade.

U.S. officials have said the leaked information exposed intelligence sources and embarrassed key officials of foreign governments. Intelligence assessments of the damage will not be presented until the sentencing phase of Manning’s court-martial.

He is expected to be sentenced to 20 years in prison for his plea to the charges related to the misuse of classified information. He is set to stand trial in June and will face charges including aiding the enemy and violating the Espionage Act. A conviction probably would result in a life sentence.

Manning said computer glitches in Iraq led him to back up certain files. Those workarounds later helped teach him to easily copy data onto CDs. On Jan. 8, 2010, he stuck a CD with classified information into a pocket and took it to his housing unit at a Baghdad base, where he used his personal computer to transfer the data to a memory card.

Later that month, while on leave in Massachusetts, Manning said he confided in a man he was dating about his desire to disclose classified data. The man tried to be “supportive,” Manning said, but their relationship was fraying.

Staying with an aunt in the Washington area as a blizzard blanketed the region, Manning said he called The Post, seeking a journalist willing to examine documents detailing security incidents in Iraq. He said he spoke to a female reporter who didn’t seem to take him seriously because she said she would have to discuss any information with a senior editor.

Manning then left a message on an answering machine for the ombudsman at the Times but never heard back, he testified. Spokespeople for The Post and the Times said Thursday that the newspapers had no knowledge about any attempts by Manning to offer information.

Undeterred, he decided to approach WikiLeaks, which he said he admired for its efforts to expose the inner workings of the U.S. military. When he submitted documents to the group’s Web site the following month, he said, he chose the material carefully.

“I felt a sense of relief,” he said, adding that it gave him a “clear conscience.”

Noting that WikiLeaks had an interest in secrets about Iceland, Manning searched classified computer systems for cables about the country. He found one showing that Iceland was being “diplomatically bullied” by Washington, the private said. He sent that cable to WikiLeaks, which posted it online.

Days later, he tracked down a military video of an attack by U.S. helicopters in Baghdad that killed 12 people, including children and two employees of Reuters news agency. He sent the clip to WikiLeaks, saying he thought the military was trying to play down a mistake. He later sent classified assessments of detainees at the U.S. military prison at Guantanamo Bay, Cuba, intelligence memos and hundreds of thousands of State Department cables.

“The more I read the cables, the more I came to the conclusion that this type of information should become public,” he said.

Manning said an online relationship he struck up with a person he assumed was Assange while he was feeding WikiLeaks information offered a reprieve from a hostile environment in the Army. Both men used pseudonyms during daily online chats, he said, that covered personal issues, including Manning’s feelings of alienation while he shared a small housing unit with a comrade who appeared to dislike gays. “I looked forward to conversations,” he said, because they “allowed me to be myself.”

Manning was arrested in Iraq in May 2010 after he was identified as the leaker by a former hacker in whom the young soldier had confided over the Internet. He was later transferred to the military jail at Quantico, where his attorneys say his long solitary confinement was abusive.