He said he was disturbed by the conduct of the wars in Afghanistan and Iraq and the way American troops treated the populace. He said he did not believe the release of the information he downloaded onto a thumb drive would harm the U.S.
"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.
Manning said he was appalled by 2007 combat video of an assault by a U.S. helicopter that killed 11 men, including a Reuters news photographer. 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 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 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.
Times spokeswoman Eileen Murphy said Thursday of the purported phone call: "This is news to us."