* Court-martial of U.S. soldier opens over WikiLeaks
* Release of classified documents in 2010 angered U.S. government
Military prosecutors said arrogance drove the U.S. soldier who went on trial on Monday accused of the biggest leak of classified information in U.S. history through the WikiLeaks anti-secrecy website three years ago.
But at the opening of the court-martial of Private First Class Bradley Manning, 25, his defense 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.
The Manning case has pitted civil liberties groups who want more transparency in military and diplomatic affairs against the government, which accuses Manning of endangering lives and damaging diplomacy by leaking classified information.
Manning, a former intelligence analyst, faces a possible life sentence without parole if convicted at his court-martial in Fort Meade, Maryland, for leaking more than 700,000 secret documents in 2010.
"This is a case of what happens when arrogance meets access to classified networks," lead prosecutor U.S. Army Captain Joe Morrow said in his opening statement. "This had great interest to our adversaries and to our enemies."
The slightly built Manning, wearing dress uniform, sat between his lawyers at the defense table. He faces 21 counts, including the most serious one of aiding the enemy, and prosecution under the Espionage Act of 1917. Manning pleaded guilty in February to 10 lesser charges, but prosecutors rejected the pleas and are pursuing their original charges.
"NAIVE, BUT GOOD INTENTIONED"
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."
Coombs said 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 U.S. convoy that left U.S. personnel unhurt but wounded four members of an Iraqi family and killed one was a catalyst for Manning, Coombs said.
He felt the material he had access to should be made public, "showing the true nature of 21st-century asymmetric warfare," Coombs said.
Manning believed the material he released would not harm U.S. interests as it lacked operational value. He thought that a U.S. military video showing an 2007 Apache gunship attack that killed 12 people in Baghdad, including two Reuters staffers, had already been made available to journalists, Coombs said.
Morrow told the court Manning downloaded 251,287 State Department cables at the rate of more than 1,000 per hour.
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 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 defense questioning, Specialist Eric Baker, Manning's roommate in Iraq, described him as loner who spent most of his time on a computer.
JUDGE TO CLOSE PARTS OF TRIAL
The judge, Colonel Denise Lind, said last month she would close parts of the trial to the public to protect classified material.
Manning's court-martial is expected to run until at least late August.
Lind began the trial by asking Manning procedural questions, including whether he was willing to have the case decided by a judge rather than a jury and whether he was satisfied with his defense team.
"Yes, your honor," replied Manning, who was arrested in May 2010 while serving in Iraq. He sat listening to the proceedings through much of the day with his chin resting on his fist.
Under a ruling last month by the judge, Manning would have any sentence reduced by 112 days to compensate for the harsh treatment he received during his confinement at the Quantico Marine Base in Virginia.
The courtroom, which can seat about 40 people, was crowded with media and onlookers. They included Cornel West, a civil rights and political activist who has taught at Yale, Harvard, Princeton and the Union Theological Seminary in New York.
"I'm here to have solidarity with my devoted brother Bradley Manning," West said outside the courtroom. "He is a courageous young brother."