Bradley Manning released video of US soldiers shooting at unarmed civilians in Iraq.
The US Army private accused of leaking large numbers of classified documents to Wikileaks is due to face court martial in Fort Meade, Maryland.
Pte Bradley Manning allegedly sent 250,000 diplomatic cables and 500,000 battlefield reports from Afghanistan and Iraq to Wikileaks in 2009 and 2010.
He has pleaded guilty to 10 of the 22 charges against him but not to the most serious charge of aiding the enemy.
It is considered the largest-ever leak of secret US government documents.
If found guilty of the more serious charges, the 25-year-old faces a maximum term of 154 years in jail.
Pte Manning, who was arrested in May 2010 while serving in Iraq, has not denied leaking the documents.
He told a pre-trial hearing in February he divulged the documents to spark a public debate on the role of the US military and foreign policy.
However, prosecutors arguing on behalf of the US government say the leaks damaged national security and endangered American lives.
One of the leaked videos shows graphic footage of an Apache helicopter attack in 2007 that killed a dozen people in Baghdad, including a Reuters photographer.
"The most alarming aspect of the video to me was the seemingly delightful bloodlust the aerial weapons team happened to have," he said at the pre-trial hearing, comparing the troops to children "torturing ants with a magnifying glass".
Dozens of witnesses are expected to testify at the trial.
He will get a 112-day reduction in any jail sentence he receives after a judge ruled he had suffered unduly harsh treatment during his nine-month detention after his arrest in 2010.
Pte Manning's lawyer, David Coombs, thanked supporters who had rallied outside Fort Meade ahead of his court martial.