Story highlights Manning offers his rationale for the crimes, saying he didn't mean to harm the U.S.

He has pleaded guilty to 10 of 22 charges against him

Manning did not plead guilty to the most serious charge: aiding the enemy

His court-martial is scheduled for June 3

Pfc. Bradley Manning pleaded guilty Thursday to 10 of the 22 charges against him -- but not the most serious one, "aiding the enemy" -- in what the government says is the largest leak of classified documents in the nation's history.

And, for the first time, Manning offered his rationale for the crimes.

In court, Manning detailed why and how he sent classified material to WikiLeaks, a group that facilitates the anonymous leaking of secret information through its website

He said he passed on information that "upset" or "disturbed" him, but nothing he thought would harm the United States if it became public. Manning said he thought the documents were old and the situations they referred to had changed or ended.

Reading a statement for more than an hour, Manning described his motivations, beginning with what he called "sigact tables," documents describing significant actions in Iraq and Afghanistan that he said represented the "ground reality" of both conflicts.

He said he'd become "depressed about the situation there" and made copies of the sigact tables in his secure workstation in Iraq. Then, he took them back to the United States and pondered what to do with them.

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Manning said he first called The Washington Post. He spoke to a woman who he believed was a reporter and told her the kind of material he had. After five minutes, he got the impression she wasn't taking him seriously, he said.

He said he then called The New York Times and got nothing but answering machines, so he left a message and his phone number and e-mail address, but never heard back.

Manning said he finally decided to send the documents to the WikiLeaks organization.

"I believed if the public was aware of the data, it would start a public debate of the wars," he told the court.

Manning acknowledged to the court he was not authorized to receive the classified documents he leaked and said he knows that he had other avenues through which he could have expressed his dissatisfaction.

Exposing State Department cables, military video

After he sent the documents to WikiLeaks in early 2010, Manning said earlier, he became aware of an online debate about Iceland's financial troubles and its relations with the United Kingdom. He decided to learn more about the issue, using his access to State Department cables. He said he sympathized with Iceland in the dispute and believed that Iceland was being "bullied" by the UK, and that the United States wouldn't help. So he decided to send related information to WikiLeaks

It was published to the world within hours.

At that same time, Manning said he learned about Reuters' battle with the U.S. military over video of a helicopter gunship attack on a truck carrying a Reuters news crew in Iraq. Two Reuters staffers were killed in the attack.

He said the military told Reuters that the video might not exist, but Manning had seen it. He made a copy of the video and planned to send it to Reuters when his tour ended.

Manning said the video and the behavior of the Americans involved was so disturbing, "It burdens me emotionally."

He was so upset, he decided to upload the video to WikiLeaks immediately.

In that case, Manning said, WikiLeaks did not publish the video right away.

Later, while communicating through chat rooms with a person whom he believed to be a top WikiLeaks official, he was told that the video was about to be published, and that he wouldn't be hearing much from them for a while.

"I'd have nothing but work to distract me," Manning said.

Apparently bored by his regular analyst duties and prodded what he described as a curiosity about geopolitics, he began reading the State Department cables. He decided the American public should know how its diplomats go about conducting foreign affairs.

Manning took the most widely distributed diplomatic cables and made copies for WikiLeaks

"I believed that the public release of these cables would not damage the United States, but might be embarrassing," he told the court.

The court proceedings

After Manning's guilty pleas, Army judge Col. Denise Lind asked the defendant questions to establish that he understood what he was pleading guilty to.

In addition, she reminded him that his lawyer had filed a motion to have the case dismissed on the grounds that he was denied his right to a speedy trial -- a motion that Lind denied Tuesday. By entering guilty pleas, Manning loses his right to have an appellate court consider that ruling, if he chooses to appeal.

A military lawyer who follows the case told CNN the tactic is known as a "naked plea," or a guilty plea in the absence of a plea deal. The lawyer said that by using that strategy, the defense apparently hopes the government will feel victorious about the guilty pleas Manning has entered and won't go through the effort of a trial.

The judge accepted the guilty pleas, but noted Manning could withdraw them at any time prior to sentencing. He could receive up to 20 years on those charges.

Manning said deciding to make the material public was "beyond my pay grade."

Manning has asked for Lind, instead of the military equivalent of a jury, to decide his guilt or innocence on the 12 charges to which he pleaded not guilty. The most serious remaining charge, aiding the enemy, carries the potential for a life sentence.

The U.S. military initially detained Manning in May 2010, shortly after WikiLeaks published the State Department cables. Manning was turned in by Adrian Lamo, a former hacker , whom Manning allegedly told about leaking the classified records.

In December 2011, Manning's Article 32 hearing, the military equivalent of a grand jury hearing to determine whether enough evidence existed to merit a court-martial, began. He was formally charged in February 2012.