FORT MEADE, Md. (AP) — More than three years ago, Army Pfc. Bradley Manning was arrested in Iraq and charged in the biggest leak of classified information in U.S. history.

Since then, he admitted to sending troves of material to the anti-secrecy website WikiLeaks and pleaded guilty to charges that would send him to prison for up to 20 years. The U.S. military and the Obama administration weren't satisfied, though, and pursued a charge of aiding the enemy, which carries a potential life sentence.

The trial on that most serious charge and 20 other offenses started Monday for the 25-year-old former intelligence analyst from Oklahoma. The judge took up procedural matters and opening statements were expected to begin later Monday.

It's the most high-profile case for an administration that has come under criticism for its crackdown on leakers. The six prosecutions since Obama took office is more than in all other presidencies combined.

Manning, a slightly built soldier, sat calmly in the courtroom in his dark green dress uniform as the trial began. He chose to have his court-martial heard by a judge instead of a jury. It is expected to run all summer.

In February, Manning told military judge Army Col. Denise Lind he leaked the material to expose the American military's "bloodlust" and disregard for human life in Iraq and Afghanistan. He said he did not believe the information would harm the U.S. and he wanted to start a debate on the role of the military and foreign policy.

The judge accepted his guilty plea to reduced charges for about half of the alleged offenses, but prosecutors did not did not and moved forward with a court-martial on charges including violations of the Espionage Act and the Computer Fraud and Abuse Act.

Manning's supporters hail him as a whistleblowing hero and a political prisoner. Others view him as a traitor.

About 20 Manning supporters demonstrated in the rain outside the visitor gate at Fort Meade. They waved signs reading "free Bradley Manning" and "protect the truth" while chanting "What do want? Free Bradley. When do we want it? Now."

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U.S. officials have said the more than 700,000 Iraq and Afghanistan battlefield reports and State Department cables sent to WikiLeaks endangered lives and national security.

The material WikiLeaks began publishing in 2010 documented complaints of Iraqi detainee abuses; a U.S. tally of civilian deaths in Iraq; and America's weak support for the government of Tunisia — a disclosure Manning supporters said encouraged the popular uprising that ousted the Tunisian president in 2011 and helped trigger the Middle Eastern pro-democracy uprisings known as the Arab Spring.

Last month, the government agreed to accept Manning's guilty plea for a lesser version of one count, involving a single diplomatic cable summarizing U.S. embassy discussions with Icelandic officials about the country's financial troubles.

Manning also acknowledged sending WikiLeaks unclassified video of a 2007 U.S. Apache helicopter attack that killed civilians, including a Reuters photographer. An internal military investigation concluded the troops reasonably mistook the camera equipment for weapons; WikiLeaks dubbed the video "Collateral Murder."

The release of the cables and video embarrassed the U.S. and its allies. The Obama administration has said it threatened valuable military and diplomatic sources and strained America's relations with other governments, but the specific amount of damage hasn't been publicly revealed and probably won't be during the trial.

Lind ruled the extent of any damage is irrelevant. Defense attorney David Coombs contends it was minimal.

Much of the evidence is classified, which means large portions of the trial are likely to be closed to reporters and the public.

Lead prosecutor Maj. Ashden Fein told Lind in February that more than half of the government's 141 anticipated witnesses would testify about classified information, which would close up to 30 percent of the trial.

The judge tested alternatives to closing the courtroom, such as using code words and unclassified summaries, but Lind said it didn't work.

Prosecutors revealed plans earlier this year to call a member of the Navy SEAL team that raided Osama bin Laden's compound in 2011. He was to testify in closed court, in disguise, that he found digital evidence indicating the al-Qaida leader saw some of the material Manning released.

He will likely be scratched, though, if Lind accepts an agreement the lawyers announced last month to offer the bin Laden evidence without the testimony.

The court-martial's high degree of secrecy, including refusals to promptly release even routine filings and rulings, has fueled protests by Manning supporters. The Bradley Manning Support Network says it has raised more than $1.1 million for his defense and public outreach.

Supporters include documentary filmmaker Michael Moore, musician Graham Nash, actor John Cusack and Pentagon Papers leaker Daniel Ellsberg.

Ellsberg, a former military analyst, has said Manning's disclosures may be more significant than his own leak of a top-secret history of the Vietnam War expansion in 1971.

Manning's case gained even more attention when human rights groups and the United Nations' chief torture investigator complained about his pretrial confinement at a Marine Corps brig in Quantico, Va.

For nine months, Manning was held alone in a windowless cell 23 hours a day, sometimes with no clothing. Brig officials said it was to keep him from hurting himself or others.

Lind ruled Manning had been illegally punished and should get 112 days off any prison sentence he receives. Manning was moved in April 2011 to less restrictive conditions at Fort Leavenworth, Kansas.

His case has also led to films. In a documentary released last month, Manning was portrayed sympathetically in "We Steal Secrets: The Story of WikiLeaks."

The film left an unflattering impression of WikiLeaks founder Julian Assange, who is the subject of a separate federal investigation into whether he can be prosecuted for publishing the information Manning leaked.

Manning told the court he corresponded online with someone he believed to be Assange but never confirmed the person's identity.

WikiLeaks has been careful never to confirm or deny Manning was the source of the documents.

Assange has been holed up in the Ecuadorean Embassy in London to avoid extradition to Sweden on sex-crimes allegations.