FORT MEADE, Md. – Bradley Manning, the Army private arrested in the biggest leak of classified material in U.S. history, pleaded guilty Thursday to charges that could send him to prison for 20 years, saying he was trying to expose the American military’s “bloodlust” and disregard for human life in Iraq and Afghanistan.

Military prosecutors said they plan to move forward with a court-martial on 12 remaining charges against him, including aiding the enemy, which carries a potential life sentence.

“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,” the 25-year-old told a military judge.

He added: “I wanted the public to know that not everyone living in Iraq were targets to be neutralized.”

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.

The slightly built soldier from Crescent, Okla., read from a 35-page statement through his wire-rimmed glasses for more than an hour. The former intelligence analyst in Baghdad spoke quickly and evenly, showing little emotion even when he described how troubled he was by what he had seen.

The judge, Col. Denise Lind, accepted his plea to 10 charges involving illegal possession or distribution of classified material. He will not be sentenced until his court-martial on the other charges is over.

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. WikiLeaks posted some of the material, embarrassing the U.S. and its allies.

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.

Manning said he was appalled by 2007 combat video of a U.S. helicopter assault by 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.”