A courtroom sketch depicts Private First Class Bradley Manning, 25, (L) and his attorney David Coombs during the first day of Manning’s trial at Fort Meade in Maryland, June 3, 2013. (Reuters)

A military prosecutor charged Monday that Army Pfc. Bradley E. Manning “harvested” a massive trove of classified information from secure networks and made it available to America’s enemies knowing he would cause harm.

“This is a case about a soldier who harvested hundreds of thousands of documents and dumped them on the Internet where they would be available to the enemy,” Capt. Joe Morrow said at the opening of Manning’s court-martial at Fort Meade.

He said Manning “knew, based on his training,” that he “would put the lives of fellow soldiers at risk.”

Detained by the military since his arrest in Iraq in 2010, Manning is the central figure in the latest and most prominent in a series of leak prosecutions under the Obama administration. After months of pretrial hearings, motions and disputes over the use of classified information and public access to proceedings, the trial began in a spare courtroom behind guarded gates at Fort Meade, an Army installation 27 miles northeast of Washington

Manning, 25, is accused of passing more than 700,000 government and military files to the anti-secrecy Web site WikiLeaks, the largest leak of classified documents in U.S. history. The material, which was widely disseminated, included videos of airstrikes that killed civilians, sensitive diplomatic cables, and military reports from Iraq and Afghanistan.

He faces 22 charges, including a count of aiding the enemy, which could send him to prison for life without parole. He is also charged with violating the Espionage Act, a 1917 law created to try spies and traitors, which carries severe penalties.

The prosecutor said the judge would gain a sense of an intelligence analyst who was leaking documents to WikiLeaks almost from the moment he arrived at Forward Operating Base Hammer in Iraq in November 2009. He produced a slide show outlining “key evidence” that the prosecution intends to present, including information from an external hard drive of Manning’s personal computer and chat logs.

Morrow said Manning would “package” information and transmit it within a “couple of hours.” The prosecutor concluded his opening remarks by saying that Osama bin Laden requested and received a copy of internal U.S. military logs of the war in Afghanistan from another member of al-Qaeda.

“Evidence will show that he knew the nature of the WikiLeaks as an organization,” Morrow said. Manning “knew the dangers of unauthorized disclosure and ignored those dangers.”

In pretrial proceedings, Manning admitted to leaking classified material to WikiLeaks, saying he intended to “spark a domestic debate over the role of the military and our foreign policy in general.” He has offered to plead guilty to 10 lesser charges relating to the misuse of classified information, which could send him to prison for 20 years.

Defense lawyer David Coombs, in a brief opening statement that followed Morrow’s, addressed Manning’s motive for leaking the information to WikiLeaks, portraying the defendant as an idealistic and naive young soldier.

Coombs described an incident that he said Manning observed when he first arrived in Iraq. A convoy had been hit by a makeshift bomb, but once Manning’s fellow soldiers knew no U.S. troops were killed, they were relieved and happy, Coombs said. When the news came that an Iraqi civilian involved in the incident had died, the mood did not change. Manning was disturbed and started to reassess the reasons he was in Iraq, Coombs said. It was then that Manning “started to struggle,” the defense lawyer said.

Coombs argued that Manning believed the information he was releasing was already basically in the public domain. For example, he said Manning knew that much of the information he leaked about assessments of detainees at Guantanamo Bay had already been released by the Pentagon.

“He knew a lot of people didn’t need to be there, held there year after year with no hope,” Coombs said. Manning also thought the information might be of value to the detainees’ attorneys or ultimately of some historical importance, the lawyer said.

Coombs portrayed Manning as a soldier who hoped to make a difference by releasing this information. He described Manning as “young, naive and good intentioned.”

Testifying Monday were two members of the Army’s Criminal Investigation Command — special agents Thomas Smith and Toni Graham — and Manning’s former roommate in Iraq, military police officer Eric Baker.

Smith, called as the first witness, testified that he helped secure the scene where Manning worked, as well as his personal living space in Iraq. Smith said he arrived at Hammer on May 27, 2010, by helicopter and seized two secure computers on which Manning generally worked.

Smith said he found writable CDs in Manning’s housing unit. In one case marked as containing an Arabic language training CD, he found Apache helicopter video that showed the deaths of two Reuters news agency employees. The CD was marked “Reuters FOIA Req” in a black permanent marker.

He said he placed all the evidence in brown paper bags and eventually put them in an evidence footlocker. He said he ran out of paper bags while collecting evidence.

In attendance at the court-martial Monday were more than 40 members of the public, including Manning’s aunt and cousin. Outside Fort Meade’s main gates, more than 30 Manning supporters protested in the rain, some of them holding signs bearing slogans such as “Free Bradley Manning” and “Does America Have a Conscience?”

Ellen Nakashima contributed to this report.