Military prosecutors have said arrogance drove a US soldier to orchestrate the biggest leak of classified information in US history through the WikiLeaks website three years ago.
Private First Class Bradley Manning has gone on trial for leaking more than 700,000 secret documents in 2010.
He faces a possible life sentence without parole if convicted at his court-martial in Fort Meade, Maryland.
His defence lawyer portrayed him as a naive young soldier who had leaked the documents, combat videos and other data because he wanted to reveal the human cost of the wars in Afghanistan and Iraq.
However, lead prosecutor Captain Joe Morrow said: "This is a case of what happens when arrogance meets access to classified networks.
"This had great interest to our adversaries and to our enemies."
Pte Manning, a former intelligence analyst, faces 21 counts, including the most serious one of aiding the enemy, and prosecution under the US Espionage Act of 1917.
He pleaded guilty in February to ten lesser charges, but prosecutors rejected the pleas and are pursuing their original charges.
Pte Manning's lawyer David Coombs described his client as a humanist, "placing people first, placing value on human lives."
He called the soldier "young, naive, but good-intentioned".
Mr Coombs said Pte Manning, who is gay, was struggling with his sexual identity when he arrived in Iraq in November 2009 and was conflicted by his exposure to war and a trove of military data.
A Christmas Eve 2009 bomb attack on a US convoy that left US personnel unhurt, but wounded four members of an Iraqi family and killed one, was a catalyst for the soldier, Mr Coombs said.
He felt the material he had access to should be made public, "showing the true nature of 21st-century asymmetric warfare," Mr Coombs said.
Pte Manning believed the material he released would not harm US interests as it lacked operational value.
He thought that a US military video showing a 2007 Apache gunship attack that killed 12 people in Baghdad, including two Reuters staffers, had already been made available to journalists, Mr Coombs said.
Cpt Morrow told the court that Pte Manning downloaded 251,287 State Department cables at the rate of more than 1,000 per hour.
Pte Manning, who has been jailed since his arrest three years ago, is charged forwarding the classified documents to WikiLeaks, which began exposing the secrets.
WikiLeaks founder Julian Assange has taken refuge in the Ecuadorean Embassy in London since June 2012 to avoid extradition to Sweden, where he is wanted for questioning about allegations of rape and sexual assault, which he denies.
The first witness in the case was Sergeant First Class Thomas Smith, a military investigator.
He seized Pte Manning's computers, disks and other equipment at the Iraq base east of Baghdad, which he described as being "in the middle of nowhere".
Under defence questioning, Specialist Eric Baker, Pte Manning's room-mate in Iraq, described him as loner who spent most of his time on a computer.
The judge, Colonel Denise Lind, said last month she would close parts of the trial to the public to protect classified material.
Pte Manning's court-martial is expected to run until at least late August.
