He addressed the court on a day of often emotional testimony from family members about his troubled childhood and from a psychologist who said Manning felt extreme mental pressure in the "hyper-masculine" military because of his gender-identity disorder his feeling that he was a woman trapped in a man's body.
"I am sorry that my actions hurt people. I'm sorry that they hurt the United States," he said as he began.
The soldier said that he understood what he was doing but that he did not believe at the time that leaking a mountain of classified information to the anti-secrecy website would cause harm to the U.S.
Though he often showed little reaction to court proceedings during most of the two and a half month court-martial, Manning appeared to struggle to contain his emotions several times Wednesday during testimony from his sister, an aunt and two mental health counselors, one who treated him and another who diagnosed him with several problems.
Manning, 25, could be sentenced to 90 years in prison for the leaks, which occurred while he was working as an Army intelligence analyst in Iraq in 2010. The judge will impose the sentence, though exactly when is unclear. The next session, for any prosecution rebuttal testimony, is set for Friday.
Speaking quickly but deliberately, Manning took only a few minutes to make his statement Wednesday. He appeared to be reading it from papers he was holding and looked up a number of times to make eye contact with the judge. It was an unsworn statement, meaning he could not be cross-examined by prosecutors.
He said he realizes now that he should have worked more aggressively "inside the system" to draw attention to his concerns about the way the war was being waged. He said he wants to get a college degree, and he asked for a chance to become a more productive member of society.
His conciliatory tone was at odds with the statement he gave in court in February, when he condemned the actions of U.S. soldiers overseas and what he called the military's "bloodlust."
Defense attorney David Coombs told Manning supporters that Manning's heart was in the right place.
"His one goal was to make this world a better place," Coombs said.
Manning's apology could carry substantial weight with the military judge, said Eugene Fidell, who teaches military justice at Yale.
"He faces extraordinarily long confinement and if he is coming across subjectively as contrite, I think that may do him some real good with the sentencing," Fidell said.
Manning's attorneys contend he showed clear signs of deteriorating mental health before and during his deployment that should have prevented commanders from sending him to a war zone to handle classified information.
Manning eventually came out to Capt. Michael Worsley, emailing the clinical psychologist a photo of himself in a long, blond wig and lipstick. The photo was attached to a letter titled "My problem," in which Manning described his internal struggle and said he had hoped that a military career would "get rid of it."
Worsley testified Wednesday that the soldier was struggling under extreme conditions.
"You put him in that kind of hyper-masculine environment, if you will, with little support and few coping skills, the pressure would have been difficult to say the least," Worsley said.
Worsley's testimony portrayed some military leaders as lax at best and obstructionist at worst when it came to tending to soldiers with mental health problems.
"I questioned why they would want to leave somebody in a position with the issue they had," Worsley said.
Navy Capt. David Moulton, a psychiatrist who spent 21 hours interviewing Manning at Fort Leavenworth, Kan., after his arrest, testified as a defense witness that Manning's gender identity disorder, combined with narcissistic personality traits, idealism and his lack of friends in Iraq, caused him to conclude he could change the world by leaking classified information.
He said Manning was struggling to balance his desire to right wrongs with his sense of duty to complete his Army tasks and his fear of losing his GI benefits and the opportunity to attend college.
"His decision-making capacity was influenced by the stress of his situation for sure," Moulton said.
Moulton also reported for the first time in open court that Manning has symptoms of fetal alcohol syndrome and Asperger syndrome, which is an autism spectrum disorder.
Also Wednesday, Manning's sister Casey Major, 36, testified that they grew up with two alcoholic parents in a rural home outside Crescent, Okla. She said their mother attempted suicide with a Valium overdose after Brian Manning left his wife when Bradley Manning was 12.
After looking tearfully at a series of childhood photographs presented by defense attorney David Coombs, Major said Manning has matured since his arrest.
"I just hope he can be who he wants to be. I hope he can be happy," she said. After the court went into recess, Manning went to his sister, hugged her and said something while touching his right hand to his heart.
At least 46 international journalists and 78 spectators were in attendance. Many spectators wore black "Truth" T-shirts.