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Patrick Semansky

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.

"We were risking so much for people who seemed unwilling to cooperate with us," said the 25-year-old soldier, who worked as an intelligence analyst in Iraq. It was spawning "frustration and hatred on both sides," he said.

He is expected to be sentenced to 20 years in prison after his conviction on charges related to the misuse of classified information. He is set to stand trial in June on 12 more serious charges, including aiding the enemy and espionage. A conviction probably would result in a life sentence.