It was the first time Manning directly admitted leaking the material to the anti-secrecy website WikiLeaks and detailed the frustrations that led him to do it.
Sitting before a military judge, the slightly built 25-year-old soldier from Oklahoma read from a 35-page statement through his wire-rimmed glasses for more than an hour. He spoke quickly and evenly, showing little emotion even when he described how troubled he was by what he had seen.
"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," Manning said.
A military judge, Col. Denise Lind, is weighing whether to accept Manning's guilty plea to reduced charges on 10 counts.
Even then, military prosecutors can still pursue a court-martial on the remaining 12 charges. One of those is aiding the enemy, which carries a possible life sentence. Prosecutors haven't disclosed their plans.
Manning said he didn't think the information would harm the U.S. and he decided to release it because he was disturbed by the conduct of the wars in Afghanistan and Iraq and the seeming disregard by American troops for the lives of ordinary people.
"I felt we were risking so much for people who seemed unwilling to cooperate with us, leading to frustration and hatred on both sides," he said. "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 added: "I wanted the public to know that not everyone living in Iraq were targets to be neutralized."
Manning admitted sending hundreds of thousands of Iraq and Afghanistan battlefield reports, State Department diplomatic cables, other classified records and two battlefield video clips to WikiLeaks in 2009 and 2010 while working as an intelligence analyst in Baghdad.
The battlefield reports were the first documents Manning decided to leak. He said he sent them to WikiLeaks after contacting The Washington Post and The New York Times. He said he felt a reporter at the Post didn't take him seriously, and a message he left for news tips at the Times was not returned.
Manning said he was appalled by a 2007 combat video of an aerial assault by a U.S. helicopter that killed 11 men, including a Reuters news photographer and his driver. The Pentagon concluded the 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," Manning said, adding that the soldiers' actions "seemed similar to a child torturing ants with a magnifying glass."
As for the sensitive State Department cables, he said they "documented backdoor deals and criminality that didn't reflect the so-called leader of the free world."
"I thought these cables were a prime example of the need for a more open diplomacy," Manning said. "I believed that these cables would not damage the United States. However, I believed these cables would be embarrassing."
The Obama administration has said releasing the information threatened valuable military and diplomatic sources and strained America's relations with other governments. The administration has aggressively pursued individuals accused of leaking classified material, and Manning's is the highest-profile case.
Manning has been embraced by some left-leaning activists as a whistle-blowing hero whose actions exposed war crimes and helped trigger the Middle Eastern pro-democracy uprisings known as the Arab Spring in 2010.
WikiLeaks did not immediately return a text message for comment on Manning's statement. The group has been careful never to confirm or deny whether he was the source of the documents it has posted online.
On its Twitter feed Thursday, WikiLeaks called Manning an "alleged source" and noted that he was detailing "what he says" were his dealings with the online organization.
Associated Press Writer Raphael Satter in London contributed to this report.